feat (smartplug_usb): fully cover plugs without timers

- Mark USB timer as optional as well

Issue #4499, PR #4504

custom_components/tuya_local/devices/smartplug_usb.yaml

@@ -126,6 +126,7 @@ entities:
     dps:
       - id: 15
         type: integer
+        optional: true
         name: second
         range:
           min: 0