Add edge case tests for filename field getValue

[Copilot]

Addresses missing test coverage for edge cases when source_url is undefined or empty in the filename field's getValue function.

Changes

• Add test case for source_url undefined → expects undefined from getFilename
• Add test case for source_url empty string → expects undefined from getFilename
• Wrap getValue tests in describe block for consistency with other field tests (e.g., media_dimensions)

The tests validate the actual behavior of @wordpress/url's getFilename, which returns void (undefined) when no valid filename can be extracted, rather than an empty string.

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