Drop metadata with empty keys

[zubron]

The API currently accepts and stores metadata with empty keys. lakectl and the UI (after #9650) filter or reject these client-side, but the API should handle this for consistency across all clients and to prevent invalid metadata via direct API calls.

Expected behaviour (matches S3):

• Accept requests with empty keys (don't error)
• Drop empty keys (don't store them)

PUT: {'': 'value', 'valid': 'key'}
GET: {'valid': 'key'}              (empty key dropped)

Implementation:

• Drop keys where key is empty or whitespace only
• Request succeeds (no validation error)

Note: S3 also includes the x-amz-missing-meta header when handling these keys. I've created a separate issue for this (#9727) as it may requires additional design.