Fix $sendback for post row actions when no HTTP_REFERER is found

[mitchoyoshitaka]

On a system which doesn't properly project HTTP_REFERERs, the post row actions such as "delete", "restore", etc. will send the user back to post.php *without* the relevant post_type set, so a user can be managing pages or a custom post type and get kicked over to the general post.php.

Along the way, I discovered that the logic of wp_get_http_referer (introduced in r7438) didn't really make sense... in particular, if no HTTP_REFERER or _wp_http_referer is found, it will return an empty string instead of false. Discussed this with nacin and duck_ on irc and the correction for this is also included in the patch.

In [19272]:

Fix edit.php row action redirects when referrers are not sent. Return false from wp_get_referer() when no referrer found. Props mitchoyoshitaka. fixes #17628