Error when locking git dependency in pylock.toml format

### Describe the bug

When setting lock.format to pylock and running `pdm lock` with a dependency pointing to a branch of a GitHub repo I get the following error: 
```
tomlkit.exceptions.ConvertError: Unable to convert an object of <class 'NoneType'> to a TOML item
```

When using the old format I don't get errors.

### To reproduce

1. Create a minimal `pyproject.toml` with a Github dependency like
```toml
[project]
name = "test"
version = "0.1.0"
dependencies = [
    "pony @ git+https://github.com/henri-hulski/pony.git@stubs",
]
requires-python = ">=3.11"

[build-system]
requires = ["pdm-backend"]
build-backend = "pdm.backend"
```

2. Switch to pylock format:
```sh
pdm config lock.format pylock
```

3. Remove existing lock files.

4. Run
```sh
pdm lock -v
```

### Expected Behavior

The command should create successfully `pylock.toml`.

### Environment Information

```
PDM version:
  2.25.4
Python Interpreter:
  /home/test/test/.venv/bin/python (3.11)
Project Root:
  /home/test/test
Local Packages:
  
{
  "implementation_name": "cpython",
  "implementation_version": "3.11.2",
  "os_name": "posix",
  "platform_machine": "x86_64",
  "platform_release": "6.12.33+deb12-amd64",
  "platform_system": "Linux",
  "platform_version": "#1 SMP PREEMPT_DYNAMIC Debian 6.12.33-1~bpo12+1 (2025-07-09)",
  "python_full_version": "3.11.2",
  "platform_python_implementation": "CPython",
  "python_version": "3.11",
  "sys_platform": "linux"
}
```

### Verbose Command Output

```sh
$ pdm lock -v
pdm.termui: ======== Start resolving requirements ========
pdm.termui:   pony @ git+https://github.com/henri-hulski/pony.git@stubs
pdm.termui:   Adding requirement python>=3.11
pdm.termui:   Adding requirement pony @ git+https://github.com/henri-hulski/pony.git@stubs
pdm.termui: ======== Starting round 0 ========
unearth.vcs.git: Cloning https://github.com/henri-hulski/pony.git (revision: stubs) to /tmp/pdm-build-1rmkfj87
unearth.vcs.git: Resolved https://github.com/henri-hulski/pony.git to commit 60dbddd016d61d4a11e7c030b54bbb697f1b2dc5
pdm.termui: Running PEP 517 backend to get metadata for <Link git+https://github.com/henri-hulski/pony.git@stubs (from None)>
pdm.termui: Preparing environment(Isolated mode) for PEP 517 build...
pdm.termui: ======== Start resolving requirements ========
pdm.termui:   Adding requirement python==3.11.2
pdm.termui:   Adding requirement setuptools>=61
pdm.termui: ======== Resolution Result ========
pdm.termui:       python None
pdm.termui:   setuptools 80.9.0
pdm.termui: Fetching hashes for setuptools@80.9.0
pdm.termui: Installing setuptools@80.9.0...
pdm.termui: Using cached response for https://files.pythonhosted.org/packages/a3/dc/17031897dae0efacfea57dfd3a82fdd2a2aeb58e0ff71b77b87e44edc772/setuptools-80.9.0-py3-none-any.whl
unearth.preparer: Using cached <Link https://files.pythonhosted.org/packages/a3/dc/17031897dae0efacfea57dfd3a82fdd2a2aeb58e0ff71b77b87e44edc772/setuptools-80.9.0-py3-none-any.whl (from https://pypi.org/simple/setuptools/)>
pdm.termui: Synchronization complete.
pdm.termui: /tmp/pdm-build-env-6ydwn66p-shared/lib/python3.11/site-packages/setuptools/_distutils/dist.py:289: UserWarning: Unknown distribution option: 'test_suite'
pdm.termui:   warnings.warn(msg)
pdm.termui: /tmp/pdm-build-env-6ydwn66p-shared/lib/python3.11/site-packages/setuptools/dist.py:759: SetuptoolsDeprecationWarning: License classifiers are deprecated.
pdm.termui: !!
pdm.termui: 
pdm.termui:         ********************************************************************************
pdm.termui:         Please consider removing the following classifiers in favor of a SPDX license expression:
pdm.termui: 
pdm.termui:         License :: OSI Approved :: Apache Software License
pdm.termui: 
pdm.termui:         See https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license for details.
pdm.termui:         ********************************************************************************
pdm.termui: 
pdm.termui: !!
pdm.termui:   self._finalize_license_expression()
pdm.termui: running egg_info
pdm.termui: creating pony.egg-info
pdm.termui: writing pony.egg-info/PKG-INFO
pdm.termui: writing dependency_links to pony.egg-info/dependency_links.txt
pdm.termui: writing top-level names to pony.egg-info/top_level.txt
pdm.termui: writing manifest file 'pony.egg-info/SOURCES.txt'
pdm.termui: reading manifest file 'pony.egg-info/SOURCES.txt'
pdm.termui: reading manifest template 'MANIFEST.in'
pdm.termui: warning: no files found matching 'pony/flask/example/templates'
pdm.termui: warning: no files found matching '*.html'
pdm.termui: adding license file 'LICENSE'
pdm.termui: writing manifest file 'pony.egg-info/SOURCES.txt'
pdm.termui: /tmp/pdm-build-env-6ydwn66p-shared/lib/python3.11/site-packages/setuptools/_distutils/dist.py:289: UserWarning: Unknown distribution option: 'test_suite'
pdm.termui:   warnings.warn(msg)
pdm.termui: /tmp/pdm-build-env-6ydwn66p-shared/lib/python3.11/site-packages/setuptools/dist.py:759: SetuptoolsDeprecationWarning: License classifiers are deprecated.
pdm.termui: !!
pdm.termui: 
pdm.termui:         ********************************************************************************
pdm.termui:         Please consider removing the following classifiers in favor of a SPDX license expression:
pdm.termui: 
pdm.termui:         License :: OSI Approved :: Apache Software License
pdm.termui: 
pdm.termui:         See https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license for details.
pdm.termui:         ********************************************************************************
pdm.termui: 
pdm.termui: !!
pdm.termui:   self._finalize_license_expression()
pdm.termui: running dist_info
pdm.termui: creating /tmp/pdm-meta-61hombqm/pony.egg-info
pdm.termui: writing /tmp/pdm-meta-61hombqm/pony.egg-info/PKG-INFO
pdm.termui: writing dependency_links to /tmp/pdm-meta-61hombqm/pony.egg-info/dependency_links.txt
pdm.termui: writing top-level names to /tmp/pdm-meta-61hombqm/pony.egg-info/top_level.txt
pdm.termui: writing manifest file '/tmp/pdm-meta-61hombqm/pony.egg-info/SOURCES.txt'
pdm.termui: reading manifest file '/tmp/pdm-meta-61hombqm/pony.egg-info/SOURCES.txt'
pdm.termui: reading manifest template 'MANIFEST.in'
pdm.termui: warning: no files found matching 'pony/flask/example/templates'
pdm.termui: warning: no files found matching '*.html'
pdm.termui: adding license file 'LICENSE'
pdm.termui: writing manifest file '/tmp/pdm-meta-61hombqm/pony.egg-info/SOURCES.txt'
pdm.termui: creating '/tmp/pdm-meta-61hombqm/pony-0.7.19.dist-info'
pdm.termui: Adding new pin: python None
pdm.termui: ======== Starting round 1 ========
pdm.termui: Adding new pin: pony 0.7.19
pdm.termui: ======== Starting round 2 ========
pdm.termui: ======== Resolution Result ========
pdm.termui:   python None
pdm.termui:     pony git+https://github.com/henri-hulski/pony.git@60dbddd016d61d4a11e7c030b54bbb697f1b2dc5
Traceback (most recent call last):
  File "/home/test/.local/bin/pdm", line 8, in <module>
    sys.exit(main())
             ^^^^^^
  File "/home/test/.local/share/pdm/venv/lib/python3.11/site-packages/pdm/core.py", line 391, in main
    return core.main(args or sys.argv[1:])
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/test/.local/share/pdm/venv/lib/python3.11/site-packages/pdm/core.py", line 270, in main
    raise cast(Exception, err).with_traceback(traceback) from None
  File "/home/test/.local/share/pdm/venv/lib/python3.11/site-packages/pdm/core.py", line 265, in main
    self.handle(project, options)
  File "/home/test/.local/share/pdm/venv/lib/python3.11/site-packages/pdm/core.py", line 195, in handle
    command.handle(project, options)
  File "/home/test/.local/share/pdm/venv/lib/python3.11/site-packages/pdm/cli/commands/lock.py", line 123, in handle
    actions.do_lock(
  File "/home/test/.local/share/pdm/venv/lib/python3.11/site-packages/pdm/cli/actions.py", line 160, in do_lock
    project.lockfile.format_lockfile(result_repo, groups=groups, strategy=lock_strategy)
  File "/home/test/.local/share/pdm/venv/lib/python3.11/site-packages/pdm/project/lockfile/pylock.py", line 50, in format_lockfile
    data = converter.convert(groups)
           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/test/.local/share/pdm/venv/lib/python3.11/site-packages/pdm/formats/pylock.py", line 156, in convert
    packages.append(package_table)
  File "<frozen _collections_abc>", line 1078, in append
  File "/home/test/.local/share/pdm/venv/lib/python3.11/site-packages/tomlkit/items.py", line 1941, in insert
    value = item(value, _parent=self)
            ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/test/.local/share/pdm/venv/lib/python3.11/site-packages/tomlkit/items.py", line 138, in item
    val[k] = item(v, _parent=val, _sort_keys=_sort_keys)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/test/.local/share/pdm/venv/lib/python3.11/site-packages/tomlkit/items.py", line 138, in item
    val[k] = item(v, _parent=val, _sort_keys=_sort_keys)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/test/.local/share/pdm/venv/lib/python3.11/site-packages/tomlkit/items.py", line 212, in item
    raise ConvertError(f"Unable to convert an object of {type(value)} to a TOML item")
tomlkit.exceptions.ConvertError: Unable to convert an object of <class 'NoneType'> to a TOML item
```

### Additional Context

When running `pdm lock` with the old lock format it works fine.

### Are you willing to submit a PR to fix this bug?

- [ ] Yes, I would like to submit a PR.

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Error when locking git dependency in pylock.toml format #3582

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Uh oh!

Error when locking git dependency in pylock.toml format #3582

Description

Describe the bug

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Expected Behavior

Environment Information

Verbose Command Output

Additional Context

Are you willing to submit a PR to fix this bug?

Metadata

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Projects

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