Site Editor: Fix PHP warning in _wp_get_site_editor_redirection_url().

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.

Props akshaydhere, dilipbheda, johnbillion, rainynewt, sabernhardt, sainathpoojary, shovan_jaya, tusharaddweb, wildworks.
Fixes #63224.


git-svn-id: https://develop.svn.wordpress.org/trunk@60134 602fd350-edb4-49c9-b593-d223f7449a82

Author: peterwilsoncc

src/wp-admin/site-editor.php

@@ -31,7 +31,7 @@
  */
 function _wp_get_site_editor_redirection_url() {
 	global $pagenow;
-	if ( 'site-editor.php' !== $pagenow || isset( $_REQUEST['p'] ) || ! $_SERVER['QUERY_STRING'] ) {
+	if ( 'site-editor.php' !== $pagenow || isset( $_REQUEST['p'] ) || empty( $_SERVER['QUERY_STRING'] ) ) {
 		return false;
 	}