Fix crash in 3259, add test

[evankanderson]

Summary

Fix crash when evaluating flags with nil GoFeatureFlag interface. This is an interface of type go-feature-flag.Provider, with a nil value. The interface itself is not nil, because it has the type stored.

Fixes #3259

Change Type

Mark the type of change your PR introduces:

• Bug fix (resolves an issue without affecting existing features)
• Feature (adds new functionality without breaking changes)
• Breaking change (may impact existing functionalities or require documentation updates)
• Documentation (updates or additions to documentation)
• Refactoring or test improvements (no bug fixes or new functionality)

Testing

Added unit tests, verified they reproduced the panic, then fixed the panic.

Review Checklist:

• Reviewed my own code for quality and clarity.
• Added comments to complex or tricky code sections.
• Updated any affected documentation.
• Included tests that validate the fix or feature.
• Checked that related changes are merged.

[evankanderson]

This is the test that would fail (panic) before.

[coveralls]

coverage: 50.325% (+0.1%) from 50.189%
when pulling 1b8abac on evankanderson:3259-segfault-when-doing-role-assignment
into e2414bc on stacklok:main.

[JAORMX]

staging doesn't crash anymore. Thanks for the fix!