Code Modernization: Correct handling of null in wp_parse_str().

This fixes `parse_str(): Passing null to parameter #1 ($string) of type string is deprecated` notices on PHP 8.1, without change in behaviour.

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

The PHP native `parse_str()` function expects a string, however, based on the failing tests, it is clear there are functions in WordPress which passes a non-string – including `null` – value to the `wp_parse_str()` function, which would subsequently pass it onto the PHP native function without further input validation.

Most notable offender is the `wp_parse_args()` function which special cases arrays and objects, but passes everything else off to `wp_parse_str()`.

Several ways to fix this issue have been explored, including checking the received value with `is_string()` or `is_scalar()` before passing it off to the PHP native `parse_str()` function.

In the end it was decided against these in favor of a string cast as:
* `is_string()` would significantly change the behavior for anything non-string.
* `is_scalar()` up to a point as well, as it does not take objects with a `__toString()` method into account.

Executing a string cast on the received value before passing it on maintains the pre-existing behavior while still preventing the deprecation notice coming from PHP 8.1.

Reference: [https://www.php.net/manual/en/function.parse-str.php PHP Manual: parse_str()]

Follow-up to [5709].

Props jrf, hellofromTonya, lucatume, SergeyBiryukov.
See #53635.
Built from https://develop.svn.wordpress.org/trunk@51624


git-svn-id: http://core.svn.wordpress.org/trunk@51230 1a063a9b-81f0-0310-95a4-ce76da25c4cd

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.

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-51623';
+$wp_version = '5.9-alpha-51624';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.