"Anonymous credential has no username" on keyring get

[koterpillar]

Describe the bug
Getting existing credentials using keyring get fails with an exception stacktrace showing "Anonymous credential has no username".

To Reproduce
Steps to reproduce the behavior:

$ keyring set aaaa bbbb
Password for 'bbbb' in 'aaaa': <enter password>
$ keyring get aaaa bbbb
Traceback (most recent call last):
  File "<snip>/keyring", line 8, in <module>
    sys.exit(main())
             ^^^^^^
  File "<snip>/venvs/keyring/lib/python3.12/site-packages/keyring/cli.py", line 219, in main
    return cli.run(argv)
           ^^^^^^^^^^^^^
  File "<snip>/venvs/keyring/lib/python3.12/site-packages/keyring/cli.py", line 120, in run
    return method()
           ^^^^^^^^
  File "<snip>/venvs/keyring/lib/python3.12/site-packages/keyring/cli.py", line 132, in do_get
    getattr(self, f'_emit_{self.output_format}')(credential)
  File "<snip>/venvs/keyring/lib/python3.12/site-packages/keyring/cli.py", line 140, in _emit_plain
    if credential.username:
       ^^^^^^^^^^^^^^^^^^^
  File "<snip>/venvs/keyring/lib/python3.12/site-packages/keyring/credentials.py", line 39, in username
    raise ValueError("Anonymous credential has no username")
ValueError: Anonymous credential has no username

Expected behavior

keyring get should return the password set by keyring set.

Environment

• OS: macOS 14.6.1 (23G93)

$ pipx list | grep keyring <note: pipx>
package keyring 25.4.0, installed using Python 3.12.4
$ keyring --list-backends
keyring.backends.macOS.Keyring (priority: 5)
keyring.backends.fail.Keyring (priority: 0)
keyring.backends.chainer.ChainerBackend (priority: -1)

Additional context

Works fine with 25.3.0, even with a password set by 25.4.0.

[valorl]

Yep, it seems like the introduction of AnonymousCredentials, which throws on .username hasn't been handled properly in the CLI, as both _emit_json and _emit_plain will try to access it. (cc @zanieb)

[ffissore]

#695 contains a fix. Not sure if this goes against the design decision of introducing AnonymousCredential, but I don't have enough context to come up with something better