Fix `comment_notification_recipients` filter behavior so that it is still respected even on comments left by the post author

The code was bailing on this-is-a-comment-on-your-own-post detection, ignoring additional recipients. Now:

* Logic check is done within `wp_notify_postauthor()`
* Logic check is overridable via `comment_notification_notify_author` filter (default still false)
* The code doesn't bail on comment-on-own-post detection, but just removes the author from the array
* The code instead now bails if the recipients list is empty, so `comment_notification_recipients` works properly

props ethitter.
fixes #25699


git-svn-id: https://develop.svn.wordpress.org/trunk@26367 602fd350-edb4-49c9-b593-d223f7449a82