Arlec 6-speed fan: make preset mode optional

it seems there are some conditions where this is not returned, so allow it
to be missing for better compatibility.

Issue #541

custom_components/tuya_local/devices/arlec_6speed_fan.yaml

@@ -11,6 +11,7 @@ primary_entity:
     - id: 2
       name: preset_mode
       type: string
+      optional: true
       mapping:
         - dps_val: nature
           value: Natural
@@ -26,6 +27,8 @@ primary_entity:
           value: "Off"
         - dps_val: ordinary
           value: Normal
+        - dps_val: null
+          value: "Off"
     - id: 3
       name: speed
       type: string