cylc set: set success pathway outputs if none specified and no required outputs

[MetRonnie]

Supersedes #6505

R1 = a? => b

$ cylc set workflow//1/a

does nothing and currently produces no warning.

Make it emit the success pathway outputs, kind of like how skip mode works, which would be a:succeeded in this case.

https://cylc.discourse.group/t/cylc-set-for-a-task-with-multiple-outputs-doesnt-seem-to-do-anything/1074

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
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[wxtim]

Why not use cylc set workflow//1/a --out skip.

[MetRonnie]

This is to make the default behaviour of cylc set more sensible

[MetRonnie]

(This makes pytest show full diffs without all the extra junk that comes from running with -vv. I find the truncated diffs that pytest shows by default to be difficult to understand.)

[oliver-sanders]

LGTM, needs some docs in cylc set --help and the set mutation in the GraphQL schema.