Moebot S: make "Problem" sensor optional, defaulting to "OK"

Although this seems to have some non-problematic statuses, it is
reportedly stuck on "Tilted", which may be because it is no longer
being sent, and that was the last value. Treat it as non-persistent,
with a default value of OK, to clear non-current problems if that is
the case.

Issue #1983

custom_components/tuya_local/devices/moebot_s_mower.yaml

@@ -193,7 +193,11 @@ secondary_entities:
         name: sensor
         type: string
         icon_priority: 1
+        optional: true
+        persist: false
         mapping:
+          - dps_val: null
+            value: OK
           - dps_val: MOWER_LEAN
             value: Tilted
             icon: "mdi:angle-acute"