StreamBridge and KafkaNull

When sending KafkaNull over StreamBridge, we need to repackage the
potential null result back into KafkaNull.

core/spring-cloud-stream/src/main/java/org/springframework/cloud/stream/function/StreamBridge.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2020-2021 the original author or authors.
+ * Copyright 2020-2023 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -172,6 +172,11 @@ public final class StreamBridge implements StreamOperations, SmartInitializingSi
 			resultMessage = (Message<byte[]>) functionToInvoke.apply(messageToSend);
 		}
 
+		if (resultMessage == null
+			&& ((Message) messageToSend).getPayload().getClass().getName().equals("org.springframework.kafka.support.KafkaNull")) {
+			resultMessage = messageToSend;
+		}
+
 		resultMessage = (Message<?>) this.functionInvocationHelper.postProcessResult(resultMessage, null);
 
 		return messageChannel.send(resultMessage);