When saving options from the Settings page, include the `'ping_sites'` option in the allowed "writing" options list only when the `'blog_public'` option is `'1'`.

Fixes a PHP 8.1 and above "null to non-nullable" deprecation notice in `sanitize_option()` ([https://core.trac.wordpress.org/browser/trunk/src/wp-includes/formatting.php?annotate=blame#L4952 which happens when here] as part of [22255]):

{{{
Deprecated: explode(): Passing null to parameter #2 ($string) of type string is deprecated in .../wp-includes/formatting.php
}}}

**Explanation**

[https://developer.wordpress.org/apis/options/#writing Per the documentation], the `ping_sites` option requires the `'blog_public'` option to have a value of `'1'` and must be a `string` data type. `null` is not valid for this option.

The relationship between the 2 options shows itself in the `options-writing.php` code ([https://core.trac.wordpress.org/browser/tags/6.5.4/src/wp-admin/options-writing.php#L233 shown here] and in [4326]), as the `textarea#ping_sites` only renders when `'1' === get_option( 'blog_public' )`.

**What happens if `'blog_public'` is not `'1'`?**

The `'ping_sites'` option will not be a field on the page. Upon saving:

* HTTP POST (`$_POST`) does not include `'ping_sites'`. 
* Before this commit:
   * The [https://core.trac.wordpress.org/browser/trunk/src/wp-admin/options.php#L333 option's value was set to] `null` before being passed to `update_option()`. 
   * `update_option()` invokes `sanitize_option()`.
   * A `null` value for the `'ping_sites'` case was passed to `explode()`, which threw a deprecation notice on PHP 8.1 and above.
* With this commit, the `'ping_sites'` option is no longer included in the allow list and thus will not be passed to `update_options()` > `sanitize_option()` > `explode()`.

Follow-up to [22255], [12825], [4326], [949].

Props kitchin, SergeyBiryukov, swissspidy, devmuhib, rajinsharwar, hellofromTonya.
Fixes #59818.

git-svn-id: https://develop.svn.wordpress.org/trunk@58425 602fd350-edb4-49c9-b593-d223f7449a82

…t_mime_types().

Fixes a PHP 8.1 and above "null to non-nullable" deprecation notice in `get_available_post_mime_types()`:

{{{
Deprecated: preg_match(): Passing null to parameter #2 ($subject) of type string is deprecated in ./wp-includes/post.php on line 3395
}}}

[https://developer.wordpress.org/reference/functions/get_available_post_mime_types/#return This function is documented] to:
* Return `An array of MIME types.`
* as an array of `string`s, i.e. `string[]`.

A `null` or empty element within the returned array is not a valid MIME type. If a `null` exists in the returned array, it is the root cause of PHP throwing the deprecation notice.

This commit removes the `null` and empty elements from the returned array of MIME types. It also adds a unit test.

Follow-up to [56623], [56452].

Props nosilver4u, jrf, ironprogrammer, antpb, antonvlasenko, rajinsharwar, hellofromTonya. 
Fixes #59195.

git-svn-id: https://develop.svn.wordpress.org/trunk@58437 602fd350-edb4-49c9-b593-d223f7449a82