At least with lxml 4.8.0, I see the following:

$ python
Python 3.10.4 (main, Mar 25 2022, 00:00:00) [GCC 11.2.1 20220127 (Red Hat 11.2.1-9)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import lxml.etree as etree
>>> parser = etree.HTMLParser(recover=True, encoding="utf-8")
>>> tree = etree.fromstring(b"", parser)
>>> tree
>>> tree is None
True

But the type annotation says we'll always return an _Element:

Aside: I'm passing an HTMLParser here instead of an XMLParser. The type stub for fromstring doesn't allow that either, but again I'm not sure if that's a mistake in the annotations.

For context, I came across this whilst trying to use the stubs to typecheck this snippet of Synapse.