Fix: AttributeError when optional settings are undefined

[rodrigobnogueira]

Fixes #1328

• Add None as default value in axes_conf_check
• Add test coverage for missing settings scenario

What does this PR do?

Problem

The axes_conf_check function accessed settings without a default value, causing AttributeError when optional settings like AXES_CLIENT_STR_CALLABLE, AXES_USERNAME_CALLABLE, etc. were not explicitly defined in settings.py.

Solution

Added None as the default value to getattr(settings, callable_setting, None) in the axes_conf_check function. This aligns with the defaults defined in axes/conf.py and prevents the check from failing on undefined optional settings.

Changes

• axes/checks.py: Added default value to getattr on line 210
• tests/test_checks.py: Added test_missing_settings_no_error to verify fix

Fixes #1328 - AttributeError when starting Django server with django-axes installed but without explicitly defining all optional settings.

Checks

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?