Incorrect user post count when guest account is not linked

**Steps to reproduce**

1. Create a guest author account (guest-780)
2. Check post count for admin and guest author
3. Create one post (test-post-780) as guest author and remove admin as co-author
4. Check post count for admin and guest author

**Note**

The created post will increase the post count of the user that published the post on behalf of the guest author.

**Expected behaviour**

✅ Admin: 0 posts
✅ Guest: 1 post

**Current behaviour**

❌ Admin: 1 post
✅ Guest: 1 post

Related issue

#776

Reproduce the issue using the guest author account, admin, and test post described in the steps, then compare the post counts for both users. Read related issue #776 for context and trace how publishing on behalf of an unlinked guest author assigns the count. Done means the admin remains at 0 posts while the guest author has 1.

Written by the indexing model from the issue text.