move assert where it is needed

channels/binding/base.py

@@ -97,9 +97,9 @@ class Binding(object):
         self = cls()
         self.instance = instance
         # Check to see if we're covered
-        assert self.stream is not None
         payload = self.serialize(instance, action)
         if payload != {}:
+            assert self.stream is not None
             message = WebsocketDemultiplexer.encode(self.stream, payload)
             for group_name in self.group_names(instance, action):
                 group = Group(group_name)