Tweak triggering when no flows present. [skip ci]

[hjoliver]

Close #5079: if there are no active flows present, manually triggered tasks (by default) get all previous flow numbers.

(5079 is still an open question, but I think this is the probable solution and it was quick to test it out).

This is more of an enhancement than a bug fix, so targeting master.

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] Applied any dependency changes to both setup.cfg and conda-environment.yml.
• [x] Tests are included (or explain why tests are not needed).
• [ ] CHANGES.md entry included if this is a change that can affect users
• [ ] Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• [x] If this is a bug fix, PRs raised to both master and the relevant maintenance branch.