Fix handling of empty messages in _BusinessHost class

Author: grongierisc

src/grongier/pex/_business_host.py

@@ -189,7 +189,7 @@ def _dispatch_serializer(self,message):
             return self._serialize_pickle_message(message)
         elif (message is not None and self._is_iris_object_instance(message)):
             return message
-        elif (message is None):
+        elif (message is None or message == ""):
             return message
         else:
             # todo : decorator takes care of all the parameters, so this should never happen