[STORM-370] Add check for empty table before sorting dom in UI

To avoid ```tablesorter``` from throwing exception - requires additional check to ensure that table sorter is not invoked on empty recordset.  
Most of these changes look like additions of conditional checks, but this one looks like we changed the key in `response` that is actually being checked.

Formerly, we checked `outputStats` before sorting `#bolt-executor-table`.
After the change, we check `executorStats` before sorting `#bolt-executor-table`.

Was the key incorrect before the change?