Fix JsonSerializer returning None and raising TypeError on deserialize

[davexat]

Problem

JsonSerializer is broken since the 3.0 serializer refactor (#3046):

• JsonSerializer().serialize(topic, headers, data) always returns None
• JsonSerializer().deserialize(topic, headers, data) raises
  TypeError: the JSON object must be str, bytes or bytearray, not NoneType

This silently corrupts messages produced with value_serializer=JsonSerializer() and crashes consumers using value_deserializer=JsonSerializer().

Root cause

kafka/serializer/json.py inherits from Serializer, Deserializer, whose abstract methods in kafka/serializer/abstract.py are no-ops (pass). The super().serialize(...) / super().deserialize(...) calls in JsonSerializer therefore return None. Before the 3.0 refactor, JsonSerializer inherited from DefaultSerializer, which actually performs the UTF-8 encode/decode.

Fix

Restore delegation to DefaultSerializer by inheriting from it:

class JsonSerializer(DefaultSerializer):

serialize now returns UTF-8 bytes and deserialize decodes bytes before json.loads.

Tests

Added test/test_serializer.py (no serializer unit tests existed):

• DefaultSerializer: roundtrip (utf-8 / utf-16), bytes-like passthrough (bytes / bytearray / memoryview / None), deserialize(None) -> None, rejection of non-bytes input
• JsonSerializer: roundtrip dict -> bytes -> dict, None handling, deserializing plain JSON bytes

All tests pass with the fix; the JsonSerializer tests fail against the previous code (reproducing the bug).

Verification

python -m pytest test/test_serializer.py -v
# 11 passed