Fix two bugs in printing bytes instance

Bug 1:
When `value` is None, trying to call `len(None)` throws an exception.

Bug 2:
When len(`value`) <= 100, the code currently prints b'' rather than
`value`.

Author: asdaraujo

kafka/protocol/types.py

@@ -100,7 +100,7 @@ def decode(cls, data):
 
     @classmethod
     def repr(cls, value):
-        return repr(value[:100] + b'...' if len(value) > 100 else b'')
+        return repr(value[:100] + b'...' if value is not None and len(value) > 100 else value)
 
 
 class Boolean(AbstractType):