Fix FileNotFoundError when scanning path that doesn't exist

[MetRonnie]

Fixes bug introduced by #5028 where trying to call cylc clean with a workflow that doesn't exist would throw traceback. Also running cylc scan if you didn't have a ~/cylc-run dir would also throw traceback.

Follow-up to #5065, supersedes #5082

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).
• Applied any dependency changes to both setup.cfg and conda-environment.yml.
• Tests are included
• No changelog entry (follow-up to unreleased PR)
• No docs needed
• Does not need 8.0.x PR as this bug is on master only

[oliver-sanders]

Cheers.