Support for empty string usernames is inconsistent

Describe the bug:

Storing a password with an empty string "" username in windows 64, returns None with python 3.12.

To Reproduce:

All on windows 64, set up an environment with python 3.12 and keyring 24.3.1. Set a password with an empty string username "". Retrieve the password -> None.

Expected behavior
Doing the same thing with python 3.11, the password is retrieved.

Environment

Windows 64
Python 3.12.2
Keyring 24.3.1

The issue names no files, tests, or entry points. First reproduce the empty-username case on Windows with Python 3.12 and compare it with Python 3.11, then trace the keyring backend handling of empty usernames. Done means storing and retrieving a password with an empty username behaves consistently across the reported versions.

Written by the indexing model from the issue text.