Saswell C16: make unknown dp 26 optional.

It seems this dp is not always present.
Issue #233

custom_components/tuya_local/devices/saswell_c16_thermostat.yaml

@@ -97,6 +97,7 @@ primary_entity:
           value: cooling
     - id: 26
       name: unknown_26
+      optional: true
       type: integer
 secondary_entities:
   - entity: lock