fix (mrc_magnum_floorheater): properly make dp 2 optional

Completing the fix for issue #3268

custom_components/tuya_local/devices/mrc_magnum_floorheater.yaml

@@ -20,6 +20,7 @@ entities:
             hidden: true
       - id: 2
         type: string
+        optional: true
         name: preset_mode
         mapping:
           - dps_val: AUTO