Fix Key Metrics toggle in SK Settings when answer based matrices are used

[kuasha420]

Summary

Addresses issue:

• Can't toggle "Display key metrics in dashboard" when user-defined key metrics are not set. #7441

PR Author Checklist

• My code is tested and passes existing unit tests.
• My code has an appropriate set of unit tests which all pass.
• My code is backward-compatible with WordPress 5.2 and PHP 5.6.
• My code follows the WordPress coding standards.
• My code has proper inline documentation.
• I have added a QA Brief on the issue linked above.
• I have signed the Contributor License Agreement (see https://cla.developers.google.com/).

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Code Reviewer Checklist

• Run the code.
• Ensure the acceptance criteria are satisfied.
• Reassess the implementation with the IB.
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• Ensure CI checks pass.
• Check Storybook where applicable.
• Ensure there is a QA Brief.

Merge Reviewer Checklist

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• Add a changelog message to the issue.

[github-actions]

Build files for 836fe8d have been deleted.

[hussain-t]

Nice one, @kuasha420. LGTM 👍

FYI, I have included testing the behavior with a second admin in the QAB.