Ballista: Executor must return statistics in CompletedTask / CompletedJob

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**
We cannot fix the shuffle mechanism until we have partition stats, or ShuffleReaderExec will attempt to read empty partitions, causing an error.

**Describe the solution you'd like**
Scheduler should receive partition stats and only try and read from non-empty shuffle partitions.

**Describe alternatives you've considered**
As a workaround we could write empty shuffle files for empty partitions.

**Additional context**
None

Trace the scheduler and executor paths named in the issue, including CompletedTask, CompletedJob, and ShuffleReaderExec. First determine where partition statistics are produced and passed between these components; done means the scheduler receives those statistics and avoids reading empty shuffle partitions.

Written by the indexing model from the issue text.