cylc set: log warning when no outputs specified and task has no required outputs

[MetRonnie]

R1 = a? => b

$ cylc set workflow//1/a

does nothing and currently produces no warning.

This change makes it log a warning.

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
• No changelog entry needed as minor
• No docs needed
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[MetRonnie]

I realised there was a slight mistake and the test was not quite what I wanted.

@oliver-sanders Why, for task a in

R1 = """
    a? => b
    a:x
"""

is the auto-generated completion statement (x and succeeded) or failed? This is making x optional even though it is not optional in the graph.

Shouldn't it be x and (succeeded or failed)?