allow workflow config to unset global events config

[TomekTrzeciak]

This addresses a bug whereby it is not possible to reset list-type values like mail events from the global config to empty list by overriding them in the workflow config.

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 if present).
• Tests are included (or explain why tests are not needed).
• 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.

[MetRonnie]

Thanks @TomekTrzeciak 👍 it might be January before we have a chance to look at this however