* Roomba: add last mission sensor
* Set sensor as unavailable if last mission timestamp is 0
Previously, if the `mssnStrtTm` was 0, the function would return a 1970-01-01 (Unix epoch start date). With this change, the function will return None if the timestamp is 0 and the sensor will become unavailable.
* Update last_mission property to use dt_util.utc_from_timestamp