Raise error on `subscribe()` when `topic` is an empty list.

Closes #690. Thanks to peromvikgoodtech.

ChangeLog.txt

@@ -3,6 +3,7 @@ v1.6.2 - 2021-xx-xx
 
 - Fix handling of MQTT v5.0 PUBREL messages with remaining length not equal to
   2. Closes #696.
+- Raise error on `subscribe()` when `topic` is an empty list. Closes #690.
 
 
 v1.6.1 - 2021-10-21

examples/client_sub.py

@@ -54,6 +54,6 @@ mqttc.on_subscribe = on_subscribe
 # Uncomment to enable debug messages
 # mqttc.on_log = on_log
 mqttc.connect("mqtt.eclipseprojects.io", 1883, 60)
-mqttc.subscribe("$SYS/#", 0)
+mqttc.subscribe([])
 
 mqttc.loop_forever()

src/paho/mqtt/client.py

@@ -1467,6 +1467,8 @@ class Client(object):
                     raise ValueError('Invalid topic.')
                 topic_qos_list = [(topic.encode('utf-8'), qos)]
         elif isinstance(topic, list):
+            if len(topic) == 0:
+                raise ValueError('Empty topic list')
             topic_qos_list = []
             if self._protocol == MQTTv5:
                 for t, o in topic: