Ensure Message.call is None by default

`Message` has an attribute `call` which is claimed to have type
`Optional[CallMessage]`. 

But `Message` doesn't actually ensure that `call` is initialized to a value in
`__init__`. This commit fixes that inconsistency.

discord/message.py

@@ -504,6 +504,7 @@ class Message(Hashable):
         self.application = data.get('application')
         self.activity = data.get('activity')
         self.channel = channel
+        self.call = None
         self._edited_timestamp = utils.parse_time(data['edited_timestamp'])
         self.type = try_enum(MessageType, data['type'])
         self.pinned = data['pinned']