CommandLookupTopic.advertisedListenerName may be blank

Describe the bug Some of the language bindings supply an empty string for the advertisedListenerName field of CommandLookupTopic (e.g. Java vs Golang). This confuses the broker, who then fails to apply the default advertised listener for the given server port. This, in turn, leads to an incorrect topic lookup response and a subsequent connection failure.

For background, see [PIP 95] Smart Listener Selection with Multiple Bind Addresses.

To Reproduce Steps to reproduce the behavior:

1. Configure a broker with a bind address that is associated with an advertised listener.
2. Using various language clients, send a topic lookup request to the bind address, without specifying an advertised listener. Expect the default advertised listener for that binding to be used.
3. Observe that some clients receive the correct advertised listener, others don’t.

Expected behavior Expect the binding-specific advertised listener to be automatically applied to lookup request.

Proposed Solution Enhance the broker to check for an empty string in the advertisedListenerName field. (code)

[merlimat]

It will return true if the field is set. That is irrespective of the specific value that was set.

[BewareMyPower]

done.