The field `responseTopic` of the class `Mqtt5Publish` returns the fulfilled optional with value `null`

[nicolasfara]

🐛 Bug Report

When using the publish method of the rx client I check the responseTopic of the returned Mqtt5Publish object,
and I noticed that the Optional is present, but with value "null".

🔬 How To Reproduce

Steps to reproduce the behavior:

1. Instantiate the client (using the rx version)
2. Try to publish a message with the publish method
3. Check the returned value from the publish method and check the responseTopic field.

Code sample

Environment

Where are you running/using this client?

PC

Hardware or Device?

PC

What version of this client are you using?

1.3.0

JVM version?

JDK 17

Operating System?

Linux (Manjaro)

Which MQTT protocol version is being used?

Version 5

Which MQTT broker (name and version)?

test.mosquitto.org

📈 Expected behavior

I would expect an empty optional even if the "null" string is returned by the broker (?)