GH-139: [Flight] Stop return null from MetadataAdapter.getAll(String) and getAllByte(String)

[axreldable]

What's Changed

CallHeaders has 3 implementations:

• FlightCallHeaders
• ErrorFlightMetadata
• MetadataAdapter

Before this change:
MetadataAdapter could return null from getAll(String) and getAllByte(String) when there were no values for the key, because gRPC’s Metadata.getAll() returns null in that case. This was undocumented and forced callers to null-check.

After this change:
All 3 implementations consistently return an empty iterable (never null) when the key is absent or has no values. The contract is documented on the interface and covered by tests for each implementation.

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This contains breaking changes.
MetadataAdapter.getAll(String) and getAllByte(String) return empty iterator instead of null.

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Closes #139.

[axreldable]

Please add the enhancement label.

[ennuite]

Hello @axreldable
This looks good to me, thank you for it.

I note that this also fixes an NPE in MetadataAdapter.getAllByte(String key)

[axreldable]

Hello @axreldable This looks good to me, thank you for it.

I note that this also fixes an NPE in MetadataAdapter.getAllByte(String key)

Correct, it fixes NPE for getAll(key).spliterator() in MetadataAdapter.getAllByte(String key) method.