Fix rabbitmq_user when using force on user with permissions (#41361)

On rabbitmq 3.7 using `force: yes` fails because outputs of rabbitmqctl gives an empty line

```
root@rabbitmq-vm1:~# rabbitmqctl -q list_user_permissions john

root@rabbitmq-vm1:~#
```

Provoking an error on

456af458fc/lib/ansible/modules/messaging/rabbitmq_user.py (L185)

Because `perm.split('\t')` does not find any `\t`

Signed-off-by: Sylvain Rabot <s.rabot@lectra.com>
(cherry picked from commit 8ddca3e6cfd906f0ff48a2c08410a9f82d195deb)

lib/ansible/modules/messaging/rabbitmq_user.py

@@ -152,7 +152,7 @@ class RabbitMqUser(object):
             if self.node is not None:
                 cmd.extend(['-n', self.node])
             rc, out, err = self.module.run_command(cmd + args, check_rc=True)
-            return out.splitlines()
+            return out.splitlines() if len(out.strip()) else []
         return list()
 
     def get(self):