Fix: Setting stream map property to `null` does not remove it from SCHEMA message

Ryan Whittenrequested to merge
rwhitten577/sdk:370-stream-map-null-remove-from-schema into main

Closes #370 (closed)

Thanks to TDD and checking the schema properties matches the record properties, I found another bug where flattened schemas using CustomStreamMap missed the flattening part. Added that in as well and made sure all mapper tests are passing.

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