Kafka-3832: Kafka Connect's JSON Converter never outputs a null value

[Satyajitv]

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@rhauch , have added null condition in the method convertToJsonWithEnvelope ,to return null instead of empty Envelope, when enableSchemas=true.

Would also implement a similar change to toConnectData method, based on your feedback with changes made for fromConnectData method.

let me know, if there is any problem with the implementation or my understanding about the KAFKA-3832.

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[kkonstantine]

This issue has been fixed.

Closing this PR without merging given that KAFKA-3832 was actually fixed by #6027 but thanks for taking an initial stab at this bug.