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

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git-svn-id: http://core.svn.wordpress.org/trunk@26268 1a063a9b-81f0-0310-95a4-ce76da25c4cd