fix(beca_bht006_thermostat_c): handle missing mode dp

The mode dp was already marked as optional, but it seems a recent firmware
update may have made it more common to be missing.

Issue #3455

custom_components/tuya_local/devices/beca_bht006_thermostat_c.yaml

@@ -21,6 +21,9 @@ entities:
                 value: heat
               - dps_val: "wind"
                 value: fan_only
+              - dps_val: null
+                value: null
+                hidden: true
       - id: 2
         type: string
         name: mode
@@ -74,6 +77,8 @@ entities:
                 value: cooling
               - dps_val: wind
                 value: fan
+              - dps_val: null
+                value: null
       - id: 28
         type: string
         name: fan_mode