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.
Built from https://develop.svn.wordpress.org/trunk@51652
git-svn-id: http://core.svn.wordpress.org/trunk@51258 1a063a9b-81f0-0310-95a4-ce76da25c4cd
e53ffbaf95