Fix some undefined index notices related to Comment unit tests:
* There are several places where a `$_POST` index was unchecked before setting a variable
* In `wp_notify_postauthor()`, `$comment` was being returned null, but its properties were being accessed.
* In `check_ajax_referer()`, 3 different values can be checked for nonce on `$_REQUEST`, but only 1 had an `isset()`

See #25282.


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