Fix autosave.js support for checkboxes

What does this MR do and why?

This fixes a bug related to autosave.js with restoring the checkbox values.

Bug: The current code overrides the checked checkboxes when no localstorage key is found. This resulted in setting security issues as non-confidential. See https://gitlab.slack.com/archives/CV09DAXEW/p1670254362701359.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

1. Go to Security > Vulnerability Report
2. Select a vulnerability
3. Select "Create issue"
4. The new issue form should have the confidential checkbox checked.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.