IPC writer should write validity buffer for UnionArray in V4 IPC message

[viirya]

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In V4, only null types have no validity bitmap. So in V4 message, IPC writer should still write validity buffer for UnionArray for compatibility.

C++ IPC writer:
https://github.com/apache/arrow/blob/c715bebbd89089f385c9996560866da23ea1ddda/cpp/src/arrow/ipc/writer.cc#L158-L168

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