Arlec smart button: button event should be optional

Will help with device detection for issue #3153

custom_components/tuya_local/devices/arlec_smart_button.yaml

@@ -10,6 +10,7 @@ entities:
       - id: 1
         type: string
         name: event
+        optional: true
         mapping:
           - dps_val: single_click
             value: single_click