Water valve: don't set error if no value is return

Add additional mapping if value is returned to consider there is no
error.

custom_components/tuya_local/devices/ble_water_valve.yaml

@@ -54,6 +54,8 @@ secondary_entities:
         mapping:
           - dps_val: 0
             value: false
+          - dps_val: null
+            value: false
           - value: true
   - entity: sensor
     name: Operation