Support comma-delimited string as labels in issue template (#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.
