Fix for CAP GA logic

[claudiulodro]

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:

Note: This is a hotfix.

Closes NPPM-2299

Currently when a post is distributed with one active guest author

How to test the changes in this Pull Request:

1. You'll need 2 sites connected with content distribution capabilities, and guest authors enabled with the NEWSPACK_ENABLE_CAP_GUEST_AUTHORS constant enabled.
2. Before applying this patch, create a post with only one guest author assigned. Distribute it. On the target site observe the author is e.g. "adminnewspack and My Guest Author"
3. Create another post, but this time with a real author and a guest author. Distribute it. On the target site observe the author is also e.g. "adminnewspack and My Guest Author"
4. Apply this patch. Make an update to those posts and distribute it. On the target site observe the author on the first post is e.g. "My Guest Author" and the author on the second post is still "adminnewspack and My Guest Author"

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?

[matticbot]

🎉 This PR is included in version 2.17.1 🎉

The release is available on GitHub release

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