Fix comments popup link not showing

[IanDelMar]

Description

This PR fixes the early return condition in understrap_comments_popup_link().

Motivation and Context

The comments popup link does not display "Leave a comment" if comments are open but no comments have been left.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Bug has been introduced in #1831. It was never part of a release.

Checklist:

• [x] I pulled my branch from develop.
• [x] I am submitting my pull request to develop.
• [x] I have resolved any conflicts merging this pull request would create.
• [x] I have checked that there aren't other open Pull Requests for the same update/change.
• [x] My code follows the code style of this project.
• [x] I have performed a self-review of my own code.
• [x] I have commented my code, particularly in hard-to-understand areas.
• [ ] (Optional) My change requires a change to the documentation.
• [ ] (Optional) I have updated the documentation accordingly.
• [ ] (Optional) My change requires a change to the translations.
• [ ] (Optional) I have updated the translations accordingly.
• [X] composer cs:check has passed locally.
• [x] composer lint:php has passed locally.
• [x] I have read the CONTRIBUTING document.