The `get_core_checksums()` and `wp_version_check()` functions call the PHP native `http_build_query()` function, the second parameter of which is the ''optional'' `$numeric_prefix` parameter which expects a non-nullable `string`.
A parameter being optional, however, does not automatically make it nullable.
As of PHP 8.1, passing `null` to a non-nullable PHP native function will generate a deprecation notice.
In this case, this function call yielded a `http_build_query(): Passing null to parameter #2 ($numeric_prefix) of type string is deprecated` notice.
Changing the `null` to an empty string fixes this without a backward compatibility break.
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 [18697], [25540].
Props bjorsch, kraftbj, hellofromTonya, jrf.
See #54229.
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