[execution][messagehandlers] Fix write end_data_index handling in LoopMessageHandler

The None value is not properly handled and ends up
triggering a warning while it is a valid value for
end_data_index.

This patch fixes this.

beat/backend/python/execution/messagehandlers.py

@@ -416,11 +416,17 @@ class LoopMessageHandler(MessageHandler):
     def write(self, end_data_index):
         """ Trigger a write on the output"""
 
+        end_data_index = end_data_index.decode("utf-8")
+
+        if end_data_index != "None":
             try:
                 end_data_index = int(end_data_index)
             except ValueError:
                 logger.warning("recv: wrt invalid value {}".format(end_data_index))
                 end_data_index = None
+        else:
+            end_data_index = None
+
         logger.debug("recv: wrt {}".format(end_data_index))
 
         try: