Describe the bug

The SFTP backend suffers from a typed nil pointer issue that causes nil pointer dereference panics when SFTP client creation fails and the client is subsequently reused. When
GetClient()
fails, it can leave fs.sftpclient as a typed nil (*sftp.Client(nil)) instead of nil. This typed nil passes Go's == nil checks but causes panics when methods are called on it, specifically in github.com/pkg/sftp.(*Client).nextID().

To Reproduce

Steps to reproduce the behavior:

1. Create an SFTP filesystem with invalid credentials (wrong password, nonexistent host, etc.)
2. Attempt to create a client connection using FileSystem.Client() - this will fail and set fs.sftpclient to a typed nil
3. Make a second call to FileSystem.Client() - this returns the cached typed nil client without error
4. Perform any SFTP operation that calls methods on the client (e.g., file operations, directory listing)
5. See panic: runtime error: invalid memory address or nil pointer dereference in github.com/pkg/sftp.(*Client).nextID()

Minimal Reproduction

fs := NewFileSystem(WithOptions(Options{
    Username: "baduser",
    Password: "wrongpassword",
    KnownHostsCallback: ssh.InsecureIgnoreHostKey(),
}))

auth, _ := authority.NewAuthority("sftp://baduser@nonexistent-host:22")

// First call fails properly
_, err1 := fs.Client(auth)
// err1 contains connection error

// Second call returns typed nil with no error
client, err2 := fs.Client(auth) 
// err2 is nil, but client is *sftp.Client(nil)

// Any operation on client will panic
client.ReadDir("/") // PANIC: nil pointer dereference

Expected behavior

• Failed client creation should not leave typed nil pointers in the filesystem state
• Subsequent calls to Client()
  should either return a valid client or a proper error
• No panics should occur during normal error handling scenarios
• The Client() method should detect both nil and typed nil pointers correctly

Additional context

• Root Cause: Go's interface nil semantics - *sftp.Client(nil) != nil when stored in an interface
• Affected Operations: Any SFTP operation after a failed connection attempt
• Fix: Check for typed nils using reflect.ValueOf(fs.sftpclient).IsNil() in addition to == nil