Generate default ID for properties, ensure the ID is not NULL

[jhrozek]

Summary

We don't want to put the onus on the user to generate the ID.

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

as part of a larger branch

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: 53.746% (-0.008%) from 53.754%
when pulling d9fc135 on jhrozek:prop_gen_id
into 2ab6721 on stacklok:main.