Avro namespaces are optional, but required for Avrotize

[ajfabbri]

According to the Apache Avro Spec, namespaces are optional. We don't use them since some tools do not support them, and thus not using them ensures we don't have conflicting names.

Namespaces appear to be required for avrotize though:

 File "avro/python/venv/lib/python3.12/site-packages/avrotize/avrotoproto.py", line 353, in convert_avro_to_proto
    avrotoproto.convert_avro_to_proto(avro_schema_path, proto_file_path)
  File "avro/python/venv/lib/python3.12/site-packages/avrotize/avrotoproto.py", line 346, in convert_avro_to_proto
    self.avro_schema_to_proto_messages(avro_schema, proto_files)
  File "avro/python/venv/lib/python3.12/site-packages/avrotize/avrotoproto.py", line 264, in avro_schema_to_proto_messages
    self.avro_schema_to_proto_message(avro_schema, proto_files)
  File "avro/python/venv/lib/python3.12/site-packages/avrotize/avrotoproto.py", line 243, in avro_schema_to_proto_message
    file = ProtoFile({}, {}, {}, [], {}, avro_schema["namespace"])