Always store checkboxes as arrays in state

[alexluckett]

Proposed change

Always store checkboxes as arrays in state

Prevents users changing their question from radio->checkbox and submitting without valid state. This was caused by the stateschema relying on the formschema, which supports .single() to support frontends only sending a single value.

Jira ticket: https://eaflood.atlassian.net/browse/DF-686

Type of change

• Bug fix
• New feature
• Breaking change
• Misc. (documentation, build updates, etc)

Checklist

• You have executed this code locally and it performs as expected.
• You have added tests to verify your code works.
• You have added code comments and JSDoc, where appropriate.
• There is no commented-out code.
• You have added developer docs in README.md and docs/* (where appropriate, e.g. new features).
• The tests are passing (npm run test).
• The linting checks are passing (npm run lint).
• The code has been formatted (npm run format).

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[davidjamesstone]

I think this is good but I'm a little nervous simply because I can't remember why we didn't do this before. Or maybe we did and reverted.

We have this, where we convert null to [] for checkbox condition evaluation purposes, which can maybe be deleted with this change?

[jbarnsley10]

I think I'm happy with this. The DeclarationField is done in a slightly different way so won't be impacted by this change.
But it does beg the question whether we should be handling all possible question type changes, and how that impacts on existing state that might be or might not be valid anymore