Table view: sort empty entries last in ascending mode

[MetRonnie]

Closes https://github.com/cylc/cylc-ui/issues/1255

When sorting by e.g. Job ID, an absence of a value means a job has not yet been submitted by the task that this row corresponds to. So if you sort ascending, an empty value should come after actual values.

Check List

• [x] I have read CONTRIBUTING.md and added my name as a Code Contributor.
• [x] Contains logically grouped changes (else tidy your branch by rebase).
• [x] Does not contain off-topic changes (use other PRs for other changes).
• [x] Tests are included
• [x] No changelog entry included as minor