Improve unit-test coverage for ProtobufUtil

`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.

Locate the existing unit tests for ProtobufUtil and review how its public entry points are exercised. Add focused tests for testWriteToNullObject, testFromEnumClass, testEstimateSize, and the testToByteBuffer/testFromByteBuffer round-trip, then run the relevant unit-test suite to confirm the coverage additions pass.

Written by the indexing model from the issue text.