Fixing empty tags check

Right now even if you pass in an empty tags list to the module (either with
an empty string or null) it will erroneously think the tags list have changed
and re-apply the tags on every run

lib/ansible/modules/extras/messaging/rabbitmq_user.py

@@ -108,7 +108,7 @@ class RabbitMqUser(object):
         self.username = username
         self.password = password
         self.node = node
-        if tags is None:
+        if not tags:
             self.tags = list()
         else:
             self.tags = tags.split(',')