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

[artembilan]

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.