SecretService get_credential doesn't return None for missing entry

[EpicWink]

When calling keyring.get_credential using the SecretService backend, if a username is provided but the corresponding entry doesn't exist in the keyring, a Credential object with the provided username (and no password) is returned, instead of None. This is in contrast to what happens in the default implementation, where None is returns if the entry is not found.

This causes pypa/pip#8634

Reproduction

>>> import keyring
>>> c = keyring.get_credential("spam", "eggs")

Expected

>>> print(c)
None

Actual

>>> print(c)
<keyring.credentials.SimpleCredential object at 0x...>
>>> print(c.username, c.password)
eggs None

Environment

• OS: Ubuntu 20.04.1
• keyring: 21.2.1 (first seen in 19.2)
  • keyring.backends.chainer.ChainerBackend (priority: -1)
  • keyring.backends.SecretService.Keyring (priority: 5)
  • keyring.backends.fail.Keyring (priority: 0)

[mitya57]

Can you please test my PR #453?

[EpicWink]

#453 fixes the issue for me