Branch Protection: Error out if an empty branch is given

[JAORMX]

Summary

This surfaces the case where an empty branch is given to explicit branch
protection calls to GitHub. This is needed as GitHub does not properly
handle this case, and errors end up being slightly unpredictable (things
like segfaults).

Properly defaulting to non-empty branch names is handled in another PR.

Related-to: #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

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.