Passing null value for a String @MessageHeader parameter in a @PubSubClient results in a "null" String literal on the receiver side

[balapal]

Expected Behavior

When passing in a null value in @MessageHeader parameter of a @PubSubCLient method I would expect a null value on the receiver side and not a "null" String literal.

Actual Behaviour

The receiver side has "null" String literal value instead of null

Steps To Reproduce

Send a message with by calling the below send method for example: client.send("message body goes here", null);

import io.micronaut.gcp.pubsub.annotation.PubSubClient;
import io.micronaut.gcp.pubsub.annotation.Topic;
import io.micronaut.messaging.annotation.MessageHeader;
import reactor.core.publisher.Mono;

@PubSubClient
public interface ExamplePubSubClient {
    @Topic("topic-name")
    Mono<String> send(String messageContent,
                      @MessageHeader("route") String route);
}

package micronaut.reproduce;

import com.google.pubsub.v1.PubsubMessage;
import io.micronaut.gcp.pubsub.annotation.Subscription;
import io.micronaut.messaging.Acknowledgement;
import io.micronaut.messaging.annotation.MessageBody;
import io.micronaut.messaging.annotation.MessageHeader;
import reactor.core.publisher.Mono;

@io.micronaut.gcp.pubsub.annotation.PubSubListener
public class PubSubListener {
    
    @Subscription("topic-name")
    Mono<Long> onMessage(
            @MessageBody PubsubMessage messageBody,
            @MessageHeader String route,
            Acknowledgement ack
            ) {
        // I need to add this null mapping to work properly and I think this is a bug
        route = "null".equals(route) ? null : route;
        // The issue manifests also when the value is extracted from the messageBody
        var route2 = messageBody.getAttributesOrDefault("route", null);
        route2 = "null".equals(route2) ? null : route2;
        return Mono.empty();
    }
}

Environment Information

JDK 21
Micronaut Parent version: 4.6.3

Example Application

No response

Version

4.6.3

[balapal]

I suspect there is an issue with the client as both ways of reading the message header returns "null" String literal.

Also the version of listener works fine with the client that does not take (and therefore send) @MessageHeader("route") String route parameter.
Perhaps the header should be omitted by the client when the value is null (meaning it is not set)?