fix: return no posts for authors without term

[leogermani]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

When using CAP, the HPP relies on the author terms to search for posts when filtered by authors.

As described in the docs, this requires the site to have created all terms for all authors in case they are missing, running wp co-authors-plus create-author-terms-for-posts.

However, for newly created users, or for users that are not authors of any posts, terms are not created.

The end result is that if you filter by such users, there will be no filter by the author taxonomy and all posts will be returned.

This PR fixes this.

How to test the changes in this Pull Request:

1. on trunk
2. Create a new user
3. Add a HPP block anywhere and filter by this user.
4. Note that all posts are returned
5. checkout this branch
6. note that no posts are returned, as expected

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[github-actions]

Hey @leogermani, good job getting this PR merged! 🎉

Now, the needs-changelog label has been added to it.

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Thank you! ❤️

[leogermani]

cherry picked to current alphas