This fixes a `preg_match_all(): Passing null to parameter #2 ($subject) of type string is deprecated` notice on PHP 8.1.

The behavior for `null` and `string` input is covered by the existing `Tests_Functions::test_validate_file()` test.

Effect: Errors down by 238, assertions up by 1920, failures down by 1.

Props jrf, hellofromTonya, SergeyBiryukov.
See #53635.

git-svn-id: https://develop.svn.wordpress.org/trunk@51625 602fd350-edb4-49c9-b593-d223f7449a82

…` 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

… `wpdb::_real_escape()`.

The PHP native `mysqli_real_escape_string()` function expects to be passed a string as the second parameter and this is not a nullable parameter.

Passing `null` to it will result in a `mysqli_real_escape_string(): Passing null to parameter #2 ($string) of type string is deprecated` notice on PHP 8.1.

Previously, an input type check was put in place to prevent fatal errors on PHP 8.0 when an array, object or resource was passed. Changeset [48980].

A `null` value was explicitly excluded from that check, even though a `null` value being passed would only ever result in an empty string anyway.

This commit changes the previous input type check to also bow out early for `null` values and to automatically return an empty string for those.

Refs:
- https://www.php.net/manual/en/mysqli.real-escape-string.php
- https://wiki.php.net/rfc/deprecate_null_to_scalar_internal_arg

Follow-up to [48980].

Props jrf, hellofromTonya.
See #53635.

git-svn-id: https://develop.svn.wordpress.org/trunk@51799 602fd350-edb4-49c9-b593-d223f7449a82

…IncludesPlugin::test_get_plugin_files_folder()`.

The `Tests_Admin_IncludesPlugin::_create_plugin()` expects the first parameter to be a text string to be written to a plugin file using `fwrite()`.

Passing null causes a `fwrite(): Passing null to parameter #2 ($data) of type string is deprecated` notice.

Ref: https://www.php.net/manual/en/function.fwrite

Follow-up to [31002]. [41806].

Props jrf, hellofromTonya.
See #53635.

git-svn-id: https://develop.svn.wordpress.org/trunk@51800 602fd350-edb4-49c9-b593-d223f7449a82

… `_mb_substr()`.

The `_mb_substr()` function expects a string for the `$str` parameter, but does not do input validation. This function contains a `preg_match_all()` which also expects a string type for the given subject (i.e. `$str`). 

Passing `null` to this parameter results in `preg_match_all(): Passing null to parameter #2 ($subject) of type string is deprecated` notice on PHP 8.1.

To maintain the same behaviour as before, a guard clause is added to bail out early when `$str` is passed as `null`. The outcome will, in that case, only ever be an empty string.

Note: this does mean that the `_mb_substr()` function now has a subtle difference in behaviour compared to the PHP native `mb_substr()` function as the latter ''will'' throw the deprecation notice.

The existing tests already cover this issue.

Follow-up to [17621], [36017], [32364].

Props jrf, hellofromTonya.
See #53635.

git-svn-id: https://develop.svn.wordpress.org/trunk@51853 602fd350-edb4-49c9-b593-d223f7449a82

…` in `get_core_checksums()` and `wp_version_check()`.

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.

git-svn-id: https://develop.svn.wordpress.org/trunk@52019 602fd350-edb4-49c9-b593-d223f7449a82