TRAC 53635 | PHP 8.1: wp_parse_str() - fix handling of null

Fixes `parse_str(): Passing null to parameter #1 ($string) of type string is deprecated` notices on PHP 8.1.

Impact: 311 of the pre-existing tests are affected by this issue.

src/wp-includes/formatting.php

@@ -4964,7 +4964,7 @@ function map_deep( $value, $callback ) {
  * @param array  $array  Variables will be stored in this array.
  */
 function wp_parse_str( $string, &$array ) {
-	parse_str( $string, $array );
+	parse_str( (string) $string, $array );
 
 	/**
 	 * Filters the array of variables derived from a parsed string.