Notices in wp_notify_postauthor() when post_author = 0

[bpetty]

The comment AJAX tests currently fail because they fail to find a valid user account tied to the factory-generated comments while trying to send out notifications. This might still be something we would want to detect and handle in core, however, these tests shouldn't even be trying to send out notifications anyway, so this patch just disables them during these tests.

[nacin]

I think this is actually a bug in wp_notify_postauthor(). It should support post_author = 0.

[bpetty]

That's what I meant by "This might still be something we would want to detect and handle in core".

In any case, the unit tests should *still* disable comment notifications even if this did work properly. So I'd still like to see that handled.

[drozdz]

Added checking if author is not empty

[nacin]

In 27568:

Avoid notices in wp_notify_postauthor() when a post has no author.

props drozdz.
fixes #26659.