Kyvol E30 vacuum: restore optional tag to clean area dp

When PR #269 moved this to a separate sensor, the optional attribute
got removed, leading to detection issues in #338.

custom_components/tuya_local/devices/kyvol_e30_vacuum.yaml

@@ -138,6 +138,7 @@ secondary_entities:
       - id: 16
         type: integer
         name: sensor
+        optional: true
         unit: m2
         mapping:
           - scale: 10