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

• 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).
• Tests are included (or explain why tests are not needed).
• Changelog entry not included as minor
• Docs not needed

[MetRonnie]

Mixins did not make it fun to figure out where modelValue is coming from, but it's here:

cylc-ui/src/components/graphqlFormGenerator/mixins.js

Lines 23 to 37 in cb0032a

	props: {
	// the GraphQL type this input represents
	gqlType: {
	type: Object,
	required: true
	},
	// array of all GraphQL types in the schema
	types: {
	type: Array,
	default: () => []
	},
	// the value (v-model is actually syntactic sugar for this)
	modelValue: {
	required: true
	}

We shouldn't be mutating the prop, instead we should be using the model getter/setter