publish with mandatory but without routing_key leads to uninitialized warning

[abraxxa]

When a message is published to an exchange with mandatory => 1 but without any routing_key, because it's not a topic exchange, the following warning is emitted:

Use of uninitialized value in concatenation (.) or string at /home/ahartmai/.plenv/versions/5.34.0/lib/perl5/site_perl/5.34.0/Mojo/RabbitMQ/Client/Method/Publish.pm line 32.

This might also affect the functionality of the callback sub.