fix: Allow empty 'message' field in FileMessageResult (#125)

Author: marek-lewandowski

pubnub/models/consumer/pubsub.py

@@ -3,7 +3,6 @@
 
 class PNMessageResult(object):
     def __init__(self, message, subscription, channel, timetoken, user_metadata=None, publisher=None):
-        assert message is not None
 
         if subscription is not None:
             assert isinstance(subscription, str)