Branch protection remediation now uses default branch if none provided.

[blkt]

Summary

Branch protection remediation now checks if branch name is empty and gets the default branch if available.
If both are unavailable, remediation is skipped.

Co-authored-by: Juan Antonio Osorio ozz@stacklok.com
Fixes #3430

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

Manually tested.

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.