fix TopicTabooPlugin for anonymous users

Thanks to https://github.com/JNevrly
See also njouanin/hbmqtt#151

hbmqtt/plugins/topic_checking.py

@@ -27,12 +27,11 @@ async def topic_filtering(self, *args, **kwargs):
         if filter_result:
             session = kwargs.get('session', None)
             topic = kwargs.get('topic', None)
-            if topic:
-                if session.username != 'admin' and topic in self._taboo:
-                    return False
+            if session.username and session.username == 'admin':
                 return True
-            else:
+            if topic and topic in self._taboo:
                 return False
+            return True
         return filter_result