Yup, as long as the underlying data structure is an array of length numTasks, null is as good as any other "missing output" flag.  Changing to a Map containing only valid locations instead of an array might make sense if the need to get the output statuses grows beyond the present cases in MapOutputTracker and DAGScheduler (and if the performance difference is acceptable), but now that we're handling the nulls properly in those existing cases, we're good at least for now.