Avatto curtain+light: make light optional

Although this is what distinguishes the device from a plain curtain, the device
sometimes does not report the light dp on initial connection, so it cannot be detected
correctly.  Allow the dp to be missing so this config shows up alongside the plain
curtain that matches, so that it can work when the light status comes through later.

Issue #987

custom_components/tuya_local/devices/avatto_curtain_light.yaml

@@ -31,6 +31,7 @@ secondary_entities:
       - id: 102
         type: boolean
         name: switch
+        optional: true
   - entity: select
     name: Calibration
     icon: "mdi:curtains"