Handle nil values from Trusty API

[eleftherias]

Summary

Handle possible nil values in the activity and provenance scores returned by the Trusty API.
The Trusty team made a change today that allowed nil values in the response, while previously they would alway return a float.

Change Type

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• 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

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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: 53.395% (-0.01%) from 53.405%
when pulling 8313245 on eleftherias:trusty-nil-score
into d353230 on stacklok:main.