Remove getting message on launch finish, since there is no such field in this response.

Author: HardNorth

reportportal_client/aio/client.py

@@ -446,7 +446,7 @@ async def finish_launch(
         if not response:
             return None
         logger.debug("finish_launch - ID: %s", await await_if_necessary(launch_uuid))
-        return ""
+        return None
 
     async def update_test_item(
         self,

reportportal_client/client.py

@@ -772,7 +772,7 @@ def finish_launch(
                 return None
             logger.debug("finish_launch - ID: %s", self.__launch_uuid)
         self._log(self._log_batcher.flush())
-        return ""
+        return None
 
     def update_test_item(
         self, item_uuid: str, attributes: Optional[Union[list, dict]] = None, description: Optional[str] = None