When using compacted topics, a record with a null value (also called a tombstone record) represents the deletion of a key. To receive such messages in a @StreamListener method, the parameter must be marked as not required to receive a null value argument.

@StreamListener(Sink.INPUT)
public void in(@Header(KafkaHeaders.RECEIVED_MESSAGE_KEY) byte[] key,
               @Payload(required = false) Customer customer) {
    // customer is null if a tombstone record
    ...
}