Fix evohome to gracefully handle null schedules (#140036)

* extend tests to catch null schedules

* add fixture with null schedule

* remove null schedules for now

* fic the typing for _schedule attr (is list, not dict)

* add valid schedule to fixture

* update ssetpoints only if there is a schedule

* snapshot to match last change

* refactor: dont update switchpoints if no schedule

* add in warnings for null schedules

* add fixture for DHW without schedule