Set value of the checkbox group to `false` if there isn’t a value defined

[obliviga]

Description

This fix prevents a bug with the burial demo form where the user is unable to proceed to the next step due to the checkbox group's value being set to false, even if its value has been set.

Original issue(s)

https://github.com/department-of-veterans-affairs/va-forms-system-core/issues/593

Screenshots

https://user-images.githubusercontent.com/5934582/188240599-96732709-3b97-4878-82cd-d977c8cadc52.mov

Definition of done

• [ ] Package.json version has been updated to match Github release tag
• [ ] Documentation has been updated, if applicable
• [ ] A link to the original github issue has been provided
• [ ] No sensitive information (i.e. PII/credentials/internal URLs etc.) is captured in logging, hardcoded, or specs

[va-vsp-bot]

:tada: This PR is included in version 1.6.0 :tada:

The release is available on GitHub release

Your semantic-release bot :package::rocket: