schema: remove list default value

[oliver-sanders]

Should fix an integration failure reported in cylc/cylc-uiserver#672 (comment)

• This worked under graphene v2 but not v3 for reasons unknown.
• However, the default appears to be unnecessary which makes sense since it's List(LogRecords) not List(NonNull(LogRecords)).

Reviewer(s), please test the end-to-end functionality of this feature on the graphene v3 branches before approving.

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.

[wxtim]

• Merged with Warning triangles feature branch & master
• Manually tested warning 📐
• Read code

[oliver-sanders]

I'm going to sneak this in on one review so we can fix master and unblock the UIS PR.

@dwsutherland, if you get a chance to take a peek at this one sometime, lemme know if there's any reason we shouldn't fix it this way, cheers.