Optimize a `MessagingMessageListenerAdapter` returning `null` from the `DelegatingInvocableHandler.invoke()`

[artembilan]

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Right now the DelegatingInvocableHandler.invoke() always returns an InvocationResult even if result == null. In the end we may just have a log message like:

Async result is null, ignoring

which is not always true for the regular void @KafkaHandler method.

Changing the logic in the mention method to:

		if (result != null) {
			return new InvocationResult(result, this.handlerSendTo.get(handler),
					this.handlerReturnsMessage.get(handler));
		}
		else {
			return null;
		}

would make some optimization where we would not try to resolve any reply attempts and creation of many interim objects like CompletableFuture.

Since this would be a breaking change, the issue is scheduled for the next 4.0 major version.

[ivamly]

Hi, @artembilan!

Could I work on this issue?

Thanks!

[sobychacko]

@ivamly Yes, please go ahead.

[sobychacko]

See the comment on the latest PR. https://github.com/spring-projects/spring-kafka/pull/3710#issuecomment-2597068984

[ivamly]

I'll make another PR soon, no problem.