fix(theme): making checkbox icon checked optional

[dbanksdesign]

Description of changes

Part of the checkbox theme was not passing Output generic, making the design tokens underneath required. This fixes that.

Issue #, if available

Description of how you validated changes

Checklist

• Have read the Pull Request Guidelines
• PR description included
• Relevant documentation is changed or added (and PR referenced)
• yarn test passes and tests are updated/added
• No side effects or sideEffects field updated

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

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Latest commit: 80823d1

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