support empty .payload.After / .payload.Before in avro.decode

The avro decode processor states the following:

If a specific field is set to nil the processor won't have enough information to determine the type and will default to a nullable string.

In the case that the entire .payload.after is nil, the processor fails with the following error:

failed getting structured data: failed unmarshalling JSON from raw data: expected {} for map

This attempts to alleviate the issue, by instead returning an empty sdk.StructuredData in this scenario.

Fixes # (issue)

[x] I have followed the Code Guidelines.
[x] There is no other pull request for the same update/change.
[x] I have written unit tests.
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ConduitIO / conduit