Returning null from a custom WriteModelStrategy (as documented in https://docs.mongodb.com/kafka-connector/current/kafka-sink-postprocessors/#:~:text=Return%20null%20to%20indicate%20no%20op) throws a NullPointerException when trying to wrap the returned value in a java.util.Optional.

The fix seems trivial: Use Optional.ofNullable(value) instead of Optional.of(value)

https://github.com/mongodb/mongo-kafka/blob/122cfbe5e7fb7b1949e7f2d1298e1e0c540e6a5d/src/main/java/com/mongodb/kafka/connect/sink/MongoProcessedSinkRecordData.java#L91