PSmart T436: improve compatibility

hvac_action is not always reported by the device

Issue #2446

custom_components/tuya_local/devices/psmart_t436_thermostat.yaml

@@ -52,6 +52,7 @@ primary_entity:
     - id: 36
       type: string
       name: hvac_action
+      optional: true
       mapping:
         - dps_val: open
           value: heating