7a004ad7eb
Backport #21831. The [labels in issue YAML templates](https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/syntax-for-issue-forms#top-level-syntax) can be a string array or a comma-delimited string, so a single string should be valid labels. The old codes committed in #20987 ignore this, that's why the warning is displayed: <img width="618" alt="image" src="https://user-images.githubusercontent.com/9418365/202112642-93dc72d0-71c3-40a2-9720-30fc2d48c97c.png"> Fixes #17877.
