KIP-962 — Relax non-null key requirement in Kafka Streams

Accepted Kafka 3.7 Streams

Changes Kafka Streams left-join and outer-join operators (KStream-KStream, KStream-KTable, KStream-GlobalTable, KTable-KTable foreign-key) to no longer drop records with null keys, instead passing null to ValueJoiner for the absent side. The existing strict null-key rejection was inconsistent with the defined left/outer join semantics of retaining the record when no match is found.

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