This might be a problematic decision. But the way I justify this change is that if you are appending co-authors, there may already be a WP_User set as the author. So we don't really have to care whether one is passed or not. Because of this, we do not need to forcibly return a `false` flag since that is confusing to the caller, especially because we actually do save the guest authors which are given in the call! Instead, if the $append flag is false, we should expect that at least one user will be a WP_User. In that case, if none is passed in, then there is a mismatch of the intended authors. Because now, the `wp_posts.post_author` column will have an old `wp_users.ID` which remains set and most likely isn't the intent of the caller.

Also, returning the actual response from the DB, to make this call even more accurate in terms of what is actually happen at the DB layer.

A pure WP_User (i.e. a WP_User that IS NOT linked to a Guest Author) needs to be handled specially.

This refactor is absolutely necessary in order for all the previous fixes to work as expected. Without this fix, what happens is that when you use `get_coauthor_by` by searching with a Guest Author, if that Guest Author has a valid link to a WP_User, it is summarily ignored. Functions like `add_coauthors` expect at least one coauthor to be a valid WP_User so that the `wp_posts.post_author` column can be appropriately updated. The only case where this function is returning an expected value is when you search by the WP_User first. When it arrives at `$guest_author = $this->guest_authors->get_guest_author_by( $key, $value, $force );`, `$guest_author === false`. It is then forced to move to the switch statement to find a user via their WP_User data.

With this refactor, `get_coauthor_by` will now check if the `linked_account` attribute is set. If so, it will attempt to find the corresponding user for the Guest Account. It still gives priority to returning a Guest Author. When a Guest Author is not found, it will search for a WP_User. If found, it will also search to see if a linked Guest Author account exists. If it does, it will return that Guest Author object instead, without losing the fact that this account also has a WP_User associated with it.

I forgot to run tests on my previous commit. This satisfies the test Test_CoAuthors_Plus::test_get_coauthor_by_when_guest_authors_not_enabled which is expecting a WP_User when the plugin is not enabled.

This might be a problematic decision. But the way I justify this change is that if you are appending co-authors, there may already be a WP_User set as the author. So we don't really have to care whether one is passed or not. Because of this, we do not need to forcibly return a `false` flag since that is confusing to the caller, especially because we actually do save the guest authors which are given in the call! Instead, if the $append flag is false, we should expect that at least one user will be a WP_User. In that case, if none is passed in, then there is a mismatch of the intended authors. Because now, the `wp_posts.post_author` column will have an old `wp_users.ID` which remains set and most likely isn't the intent of the caller.

Also, returning the actual response from the DB, to make this call even more accurate in terms of what is actually happen at the DB layer.

A pure WP_User (i.e. a WP_User that IS NOT linked to a Guest Author) needs to be handled specially.

This refactor is absolutely necessary in order for all the previous fixes to work as expected. Without this fix, what happens is that when you use `get_coauthor_by` by searching with a Guest Author, any link to a WP_User the Guest Author may have is summarily ignored. Functions like `add_coauthors` expect at least one coauthor to be a valid WP_User so that the `wp_posts.post_author` column can be appropriately updated. The only case where this function is currently returning an expected value is when you search by a WP_User account/field first. When it arrives at `$guest_author = $this->guest_authors->get_guest_author_by( $key, $value, $force );`, `$guest_author === false`. It is then forced to move to the switch statement to find a user via their WP_User data.

With this refactor, `get_coauthor_by` will now check if the `linked_account` attribute is set. If so, it will then attempt to find the corresponding WP_User for the Guest Author. Crucially, it still gives priority to returning a Guest Author. When a Guest Author is not found, it will then attempt to search for a WP_User. If found, it will also search to see if a linked Guest Author account exists. If it does, it will return that Guest Author object instead, without losing the fact that this account also has a WP_User associated with it.

These user_login's were causing other tests to fail because you cannot create another user with the same user_login.

Older version of PHPUnit do not have this function available. Updating to workaround: `assertTrue( property_exists( $obj, 'prop' ) )`

Relying instead on wp_user property which has already been used before.

By fully testing CoAuthorPlus::get_coauthor_by(), we can remove some repetitive assertions that don't directly relate to what's being tested.