Improve unit-test coverage for ProtobufUtil

[vasiliy-mikhailov]

ProtobufUtil has several public entry points whose edge-case behaviour is not pinned by the current suite. Add focused, additive unit tests (no production changes) for the cases a maintainer confirmed are worth covering:

• testWriteToNullObject — writing a null object
• testFromEnumClass — reading an enum type
• testEstimateSize — size estimation
• testToByteBuffer / testFromByteBuffer — ByteBuffer round-trip

Tracked for PR #710.