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ARROW-10962: [FlightRPC][Java] fill in empty body buffer if needed

Fixes compatibility with standard Protobuf implementations, which may omit serialization of an empty field. In the case of an empty record batch, this would cause the Java client to fail a precondition check since there would be no body buffer instead of a single empty body buffer.

Closes #8963 from lidavidm/arrow-10962

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: liyafan82 <fan_li_ya@foxmail.com>