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.

Author: jrfnl

src/wp-includes/formatting.php

@@ -4944,7 +4944,13 @@ 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 );
+	if ( ! isset( $array ) ) {
+		$array = array();
+	}
+
+	if ( is_scalar( $string ) ) {
+		parse_str( $string, $array );
+	}
 
 	/**
 	 * Filters the array of variables derived from a parsed string.