Fixes a PHP Warning for an undefined array key "QUERY_STRING" in `_wp_get_site_editor_redirection_url()` in some PHP configurations.
Depending on the configuration, `$_SERVER['QUERY_STRING']` can either be unset or an empty string when no query string included in the URL. This changes the condition from a falsey check to an `empty()` check.
Reviewed by Mamaduka.
Merges [60134] to the 6.8 branch.
Props akshaydhere, dilipbheda, johnbillion, rainynewt, sabernhardt, sainathpoojary, shovan_jaya, tusharaddweb, wildworks.
Fixes#63224.
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