Fix nil:NilClass error when queue['messages'] is nil.
Fix nil:NilClass error when queue['messages'] is nil.

EslamElHusseiny/sensu-plugins-rabbitmq,DvaBearqLoza/sensu-plugins-rabbitmq,dalesit/sensu-plugins-rabbitmq,alexpekurovsky/sensu-plugins-rabbitmq,sensu-plugins/sensu-plugins-rabbitmq