Allow nil receiver on Properties

[jhrozek]

Summary

Since all the providers that do not explicitly implement properties
interface return nil, nil, it might be a good idea to implement a nil
receiver for Properties.

Change Type

• 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

make test

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.

[coveralls]

coverage: 53.892% (-0.007%) from 53.899%
when pulling 0c995bf on jhrozek:properties_nil_receive
into 02a42be on stacklok:main.