I saw some characters in 'user password' field after saving settings although I left it blank.
Please see attached image, 'consumer.jpg'.
So I opened configuration file, org.jenkinsci.plugins.rabbitmqconsumer.GlobalRabbitmqConfiguration.xml and then found a hashed value like 'onZ0hovXkMgFroQGV47AXw==' in 'userPassword' entity.

I don't know about origin of that hash value.
This plugin performs action for updating rabbitmq connection if I left 'user password' blank again because blank password is not same with saved hash value.

I think this is a minor bug and can be resolved easily.
So I'll try to solve this problem if anybody doesn't resolve this issue.