Test 'Client.list_subscriptions' w/ empty return set.

Addresses:
#1440 (comment)

Author: tseaver

system_tests/pubsub.py

@@ -118,6 +118,9 @@ def test_list_subscriptions(self):
         self.assertFalse(topic.exists())
         topic.create()
         self.to_delete.append(topic)
+        empty, _ = Config.CLIENT.list_subscriptions(
+            topic_name=DEFAULT_TOPIC_NAME)
+        self.assertEqual(len(empty), 0)
         subscriptions_to_create = [
             'new%d' % (1000 * time.time(),),
             'newer%d' % (1000 * time.time(),),