Code Modernization: Pass correct default value to http_build_query() in WP_Sitemaps_Provider::get_sitemap_url().

The `WP_Sitemaps_Provider::get_sitemap_url()` method calls the PHP native `http_build_query()` function, the second parameter of which is the ''optional'' `$numeric_prefix` parameter which expects a `string`.

A parameter being optional, however, does not automatically make it nullable.

As of PHP 8.1, passing `null` to a non-nullable PHP native function will generate a deprecation notice.

In this case, this function call yielded a `http_build_query(): Passing null to parameter #2 ($numeric_prefix) of type string is deprecated` notice.

Changing the `null` to an empty string fixes this without a backward compatibility break.

This change is already covered by tests as 14 of the existing tests failed on these function calls when running the tests on PHP 8.1.

References:
* [https://www.php.net/manual/en/function.http-build-query.php PHP Manual: http_build_query()]
* [https://wiki.php.net/rfc/deprecate_null_to_scalar_internal_arg PHP RFC: Deprecate passing null to non-nullable arguments of internal functions]

Follow-up to [48470].

Props jrf.
See #53635.

git-svn-id: https://develop.svn.wordpress.org/trunk@51652 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/sitemaps/class-wp-sitemaps-provider.php

@@ -160,7 +160,7 @@ abstract class WP_Sitemaps_Provider {
 		);
 
 		if ( ! $wp_rewrite->using_permalinks() ) {
-			$basename = '/?' . http_build_query( $params, null, '&' );
+			$basename = '/?' . http_build_query( $params, '', '&' );
 		}
 
 		return home_url( $basename );