Silvercrest kettle: assume boil operation when warm dp is missing.

DP 13 does not seem to be received from the device when it should be false.
Possibly this will also mean it needs to be set to non-persistent so it
clears when warm goes off, but we'll need to observe the behaviour after
this change to see.

Issue #2722

custom_components/tuya_local/devices/silvercrest_kettle.yaml

@@ -19,6 +19,9 @@ entities:
                 value: Keep warm
               - dps_val: true
                 value: Boil
+              - dps_val: null
+                value: Boil
+                hidden: true
       - id: 2
         type: integer
         name: current_temperature