GH-661 Fix KafkaNull as input type conversion

Basically KafkaNull gets a special treatement where if we see it as Message payload and input type of the function is Message we give it as is and if the input type is POJO then we simply return null

Resolves #661

Author: olegz

Diff for: spring-cloud-function-context/src/main/java/org/springframework/cloud/function/context/catalog/SimpleFunctionRegistry.java

@@ -853,9 +853,8 @@ else if (this.skipInputConversion) {
 						: new OriginalMessageHolder(((Message) input).getPayload(), (Message<?>) input);
 			}
 			else if (input instanceof Message) {
-				if (((Message) input).getPayload().getClass().getName().equals("org.springframework.kafka.support.KafkaNull")
-						&& !this.isInputTypeMessage()) { //TODO rework
-					return ((Message) input).getPayload();
+				if (((Message) input).getPayload().getClass().getName().equals("org.springframework.kafka.support.KafkaNull")) {
+					return FunctionTypeUtils.isMessage(type) ? input : null;
 				}
 
 				if (functionInvocationHelper != null) {