Allow null values to be set to null for jsonformat

[bjoernhaeuser]

Problem

Using the JsonFormat to write "from" debezium to kafka and then using
the s3sinkconnector to read from kafka and save to s3, causes null
values to be stored always with their default values.

Solution

Therefore adding a new config property (for backwards compatibility) to
allow the value transformer inside the s3sinkconnector to be configured
correctly.

Tests for the configuration and and integration have been added as well.

This addresses #716, but for json, instead of avro

Does this solution apply anywhere else?

• yes
• no

If yes, where?

Test Strategy

Testing done:

• Unit tests
• Integration tests
• System tests
• Manual tests

Release Plan

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