Loycco sound nightlight: mark brightness as optional.

When light is off, the device does not report brightness.  Mark as
optional to improve detection chances.

issue #1042

custom_components/tuya_local/devices/loycco_sound_nightlight.yaml

@@ -27,6 +27,7 @@ primary_entity:
     - id: 6
       name: brightness
       type: integer
+      optional: true
       range:
         min: 10
         max: 1000