samples: xbee network update: process more requests after one with no tasks

This commit fix a bug that was preventing to process update requests received
after one with no tasks.

Signed-off-by: Tatiana Leon <Tatiana.Leon@digi.com>

Author: tatianaleon

samples/xbee/demos/NetworkUpdateSample/README.md

@@ -196,7 +196,7 @@ Supported platforms
 
 License
 -------
-Copyright (c) 2021, Digi International, Inc.
+Copyright (c) 2021, 2022, Digi International, Inc.
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal

samples/xbee/demos/NetworkUpdateSample/main.py

@@ -1,4 +1,4 @@
-# Copyright (c) 2021, Digi International, Inc.
+# Copyright (c) 2021, 2022, Digi International, Inc.
 #
 # Permission is hereby granted, free of charge, to any person obtaining a copy
 # of this software and associated documentation files (the "Software"), to deal
@@ -537,6 +537,7 @@ def _process_update_request(self, update_tasks):
         """
         if not update_tasks:
             log.error("No update tasks to perform")
+            self._request = None
             return
 
         if log.isEnabledFor(logging.INFO):