YX316 aroma diffuser: make sound switch optional

The dps listing in comments at the top does not actually list it, so it should
have been optional from the start.

Issue #1134

custom_components/tuya_local/devices/yx316wifi_aroma_diffuser.yaml

@@ -94,6 +94,7 @@ secondary_entities:
       - id: 6
         type: boolean
         name: switch
+        optional: true
         mapping:
           - dps_val: true
             icon: "mdi:speaker"