In `wp_ajax_ajax_tag_search()`, bail immediately if `$_GET['tax']` isn't set so that all of the variable setting can happen in the same nest scope as the rest of the function - `wp_die()` confuses Scrutinizer.

See #30224.


git-svn-id: https://develop.svn.wordpress.org/trunk@30167 602fd350-edb4-49c9-b593-d223f7449a82