Allow empty provider in repo reconcile command

[eleftherias]

Summary

We don't need to validate the provider, since we validate the project ID.

Fixes #3223

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: 50.497% (+0.003%) from 50.494%
when pulling 87c9e12 on eleftherias:3223-repo-reconcile-no-provider
into ee66f6c on stacklok:main.