Fix using fastavro if reader schema is None

[treefrog00]

1. Don't attempt to call to_json on reader schema if it's None
   Reader schema is optional, yet currently an exception in calling reader_schema_obj.to_json() will cause the if HAS_FAST: block to raise an exception and hence a fallback to the Python avro implementation

2. Use json.loads(str(schema)) rather than schema.to_json(), due to confluent-kafka 1.0.0 is incompatible with avro-python3 1.9.0 #610

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[treefrog00]

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[rnpridgeon]

Thanks @jamesjrg , I am actually refactoring the avro module this week to eliminate the fallback behavior all together as its a bit messy and I don't believe its needed anymore. It may have been when this client was originally drafted but fastavro appears to support most if not all the things the apache distribution does without the semver headaches.

Moving forward we will provide two avro serializers, Apache and FastAvro which will operate independently of one another. The default however will be Fast Avro. The idea being that we can eventually phase out the apache module all together.

That said have I'd like to ask if you have ever run into a situation where the fallback behavior was actually needed (barring bugs in the fallback logic itself which is a bit messy)?

Thanks,
Ryan

[treefrog00]

Never needed the fallback behaviour nope! Though did at one point need to pass an extra argument to fastavro to get back union names: fastavro/fastavro@eb53076