Message value can be None

kafka/protocol/message.py

@@ -22,7 +22,7 @@ class Message(Struct):
     CODEC_SNAPPY = 0x02
 
     def __init__(self, value, key=None, magic=0, attributes=0, crc=0):
-        assert isinstance(value, bytes), 'value must be bytes'
+        assert value is None or isinstance(value, bytes), 'value must be bytes'
         assert key is None or isinstance(key, bytes), 'key must be bytes'
         self.crc = crc
         self.magic = magic