Fix TypeError in cylc message mutation form

[MetRonnie]

Bug

Results in nothing happening and this error:

TypeError: Cannot read properties of null (reading 'length')

Check List

• [x] I have read CONTRIBUTING.md and added my name as a Code Contributor.
• [x] Contains logically grouped changes (else tidy your branch by rebase).
• [x] Does not contain off-topic changes (use other PRs for other changes).
• [ ] Tests are included (or explain why tests are not needed).
• [x] Changelog entry not included as minor
• [x] Docs not needed