Use fallback key ID when key version is empty

[dmjb]

This addresses a bug preventing the decryption of old secrets when the default key has been rotated.

Summary

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Fixes #(related issue)

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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: 52.362% (+0.02%) from 52.342%
when pulling 48df42c on fix-default-algorithm-empty-string
into f8c48ba on main.