Inconsistent return type in `get_active_blog_for_user()`

[flixos90]

While writing unit tests for #38355, I discovered that get_active_blog_for_user() may possibly return a plain object like those in the array returned by get_blogs_of_user(). We should make sure that only WP_Site objects are returned.

On the other hand, this issue is not really critical since, as far as I could see, the entire clause starting with ! is_object(... will never be reached - I might have missed a use-case, but I tried several "combinations" of what could happen, but never got to the point where it was executed.

[jeremyfelt]

This statement should be reachable if the user's primary_blog happens to be set as archived, spam, or deleted.

What I don't get is that we do another call to get_blogs_of_user() and pass the second parameter to get a list of all sites, including those set as archived, spam, or deleted. We then ignore those when trying to find a new primary to set for the user. That's probably another ticket.

Quite a bit about this could be refactored, but 38430.diff​ is pretty non-intrusive and should result in a consistent WP_Site return as we already have a better object available to us.

[jeremyfelt]

In 42866:

Multisite: Return WP_Site as expected in get_active_blog_for_user().

Previously it was possible for a standard object representing the site to be returned in some cases.

Fixes #38430.