Silvercrest kettle: make timer optional.

The logs from the device do not have the timer attribute documented in the
API Explorer docs. Presumably this is only available when the timer is active,
or is a write only property.

Issue #516

custom_components/tuya_local/devices/silvercrest_kettle.yaml

@@ -32,12 +32,12 @@ primary_entity:
         max: 100
       mapping:
         - step: 5
-      # One of these unknowns should be the "16 preset programs". Both are numeric strings (so probably enums)
     - id: 13
       type: boolean
       name: warm
       optional: true
       hidden: true
+      # One of these unknowns should be the "16 preset programs". Both are numeric strings (so probably enums)
     - id: 15
       type: string
       name: unknown_15
@@ -63,6 +63,7 @@ secondary_entities:
         type: integer
         name: value
         unit: min
+        optional: true
         range:
           min: 0
           max: 120