Fix global topic prefix handling (#32)

mqtt.py

@@ -59,7 +59,7 @@ class MqttClient:
     self.mqttc.connect(self.hostname, port=self.port)
 
     for topic, callback in callbacks:
-      if self.topic_prefix is not None:
+      if self.topic_prefix:
         topic = "{}/{}".format(self.topic_prefix, topic)
       self.mqttc.message_callback_add(topic, callback)
       self.mqttc.subscribe(topic)

workers/thermostat.py

@@ -71,7 +71,7 @@ class ThermostatWorker(BaseWorker):
     return ret
 
   def on_command(self, topic, value):
-    if self.topic_prefix is not None:
+    if self.topic_prefix:
       _, device_name, method, _ = topic[len(self.topic_prefix+"/"):].split('/')
 
     thermostat = self.devices[device_name]