[WARM-REBOOT] fix issue of watchdog on simx when executing warm-reboo…

…t command (sonic-net#8132)

- Why I did it
to prevent python exception error when executing warm-reboot command on mellanox simulator platform

- How I did it
return None on the watchdog python script on cases that watchdog file is not exist

- How to verify it
warm-reboot is running well without the python error. error message will appear on log on these cases.
in order to avoid this error message we can simulate the watchdog on mellanox simulator platform

platform/mellanox/mlnx-platform-api/sonic_platform/watchdog.py

@@ -275,6 +275,9 @@ def get_watchdog():
         if device.startswith("watchdog") and is_mlnx_wd_main(device):
             watchdog_main_device_name = device
 
+    if watchdog_main_device_name is None:
+        return None
+
     watchdog_device_path = "/dev/{}".format(watchdog_main_device_name)
 
     watchdog = None