Fix failure to use fastavro when no Avro reader schema is given

[woodlee]

When MessageSerializer is initialized without a reader_schema, the to_json method call on the None value at L178 results in an exception. Since this happens in a broad try/except block that captures all exceptions, it was leading to a silent failure to use the fastavro library in such cases. This is a quick fix for that.

[ghost]

It looks like @woodlee hasn't signed our Contributor License Agreement, yet.

The purpose of a CLA is to ensure that the guardian of a project's outputs has the necessary ownership or grants of rights over all contributions to allow them to distribute under the chosen licence.
Wikipedia

You can read and sign our full Contributor License Agreement here.

Once you've signed reply with [clabot:check] to prove it.

Appreciation of efforts,

clabot

[woodlee]

[clabot:check]

[ghost]

@confluentinc It looks like @woodlee just signed our Contributor License Agreement. 👍

Always at your service,

clabot

[woodlee]

Thanks @mparry , I had not realized this was s duplicate of nos. #694 and #773 . Closing this.