Table view sorting: sort empty entries last when in ascending mode

[MetRonnie]

If you click e.g. the "Job ID" column to sort by the job ID, you get the tasks that haven't yet got a job ID showing before the tasks that do have a job ID. It should be the other way round because if you're sorting in such a way you're more likely to care about the values that are relevant, not empty values.

Note the more sensible sorting behaviour is already implemented for some of the columns e.g. "T-submit" (essentially treat empty value as Infinity).