Ensure cylc trigger does not fall back to flow=none by default

[MetRonnie]

Summary

Split off from #6370 as I think it's already a bug.

Problem 1

If you cylc trigger a task when there are no active flows (e.g. on restarting a completed workflow), it looks in the DB for the latest flow number(s). However, if the latest was flow=none, this would result in the task being triggered with flow=none as well.

Solution: choose the latest flow number(s) that is not flow=none.

Problem 2

With #6370 making it so that cylc remove will demote tasks to flow=none, you could end up with the situation where all tasks in the DB are flow=none, so again this would still result in the task being triggered with flow=none.

Solution: fall back to flow=1.

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• No dependency changes
• Tests are included
• Changelog entry included if this is a change that can affect users
• No docs needed
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[hjoliver]

Good 👍

[oliver-sanders]

Makes sense, looks good, 👍