PHP 8.1 | wp_xmlrpc_server::_insert_post(): fix passing null to non-nullable and more

The `wp_xmlrpc_server::_insert_post()` method creates a new post via `wp_insert_post()` or updates an existing one via `wp_update_post()` (which subsequently calls `wp_insert_post()`), but the default/fall-back values used in the function were not in line with the default/fall-back values used in the `wp_insert_post()` function.

The `wp_insert_post()` function does a `wp_parse_args()` (array merge) of the received arguments with the defaults.
If any of the received arguments would be `null`, this would overwrite the default value (see: https://3v4l.org/bfVlv ) and will lead to "passing null to non-nullable" deprecation notices on PHP 8.1 for certain arguments.

Unfortunately, the conditional logic within the `wp_xmlrpc_server::_insert_post()` function itself often uses an `isset()` to trigger certain code blocks., so syncing the defaults with those used in the `wp_insert_post()` function was not an option.

So for this commit, I've:
* I've updated the default/fall-back values in the `$defaults` array only for those values where this would not lead to a change in the behaviour of the function.
* I've also added a safeguard function, filtering out all remaining `null` values from the `$post_data` array before it is passed on to the `wp_insert_post()` or `wp_update_post()` functions.
    Removing those values is safe as this means that these array keys will now:
    - either be set to the default/fall-back value as defined in `wp_insert_post()`.
    - or not be set and for those values which don't have a default/fall-back value in `wp_insert_post()`, `wp_insert_post()` does an `! empty()` or `isset()` check anyway and those array keys not being defined means that the result of those checks will remain the same.

Includes removing a couple of conditions which are now redundant.

Includes removing an `expectDeprecation()` in the `Tests_Date_XMLRPC` test class, which is now no longer needed.

Fixes various errors along the lines of:
```
36) Tests_XMLRPC_wp_newPost::test_no_content
json_decode(): Passing null to parameter #1 ($json) of type string is deprecated

/var/www/src/wp-includes/kses.php:2074
/var/www/src/wp-includes/class-wp-hook.php:307
/var/www/src/wp-includes/plugin.php:205
/var/www/src/wp-includes/post.php:2835
/var/www/src/wp-includes/post.php:2720
/var/www/src/wp-includes/post.php:4066
/var/www/src/wp-includes/class-wp-xmlrpc-server.php:1683
/var/www/src/wp-includes/class-wp-xmlrpc-server.php:1347
/var/www/tests/phpunit/tests/xmlrpc/wp/newPost.php:25
/var/www/vendor/bin/phpunit:123
```

Author: jrfnl

src/wp-includes/class-wp-xmlrpc-server.php

@@ -1412,19 +1412,19 @@ protected function _insert_post( $user, $content_struct ) {
 		$defaults = array(
 			'post_status'    => 'draft',
 			'post_type'      => 'post',
-			'post_author'    => null,
-			'post_password'  => null,
-			'post_excerpt'   => null,
-			'post_content'   => null,
-			'post_title'     => null,
-			'post_date'      => null,
-			'post_date_gmt'  => null,
+			'post_author'    => 0,
+			'post_password'  => '',
+			'post_excerpt'   => '',
+			'post_content'   => '',
+			'post_title'     => '',
+			'post_date'      => '',
+			'post_date_gmt'  => '',
 			'post_format'    => null,
 			'post_name'      => null,
 			'post_thumbnail' => null,
-			'post_parent'    => null,
-			'ping_status'    => null,
-			'comment_status' => null,
+			'post_parent'    => 0,
+			'ping_status'    => '',
+			'comment_status' => '',
 			'custom_fields'  => null,
 			'terms_names'    => null,
 			'terms'          => null,
@@ -1499,11 +1499,11 @@ protected function _insert_post( $user, $content_struct ) {
 			$post_data['post_author'] = $user->ID;
 		}
 
-		if ( isset( $post_data['comment_status'] ) && 'open' !== $post_data['comment_status'] && 'closed' !== $post_data['comment_status'] ) {
+		if ( 'open' !== $post_data['comment_status'] && 'closed' !== $post_data['comment_status'] ) {
 			unset( $post_data['comment_status'] );
 		}
 
-		if ( isset( $post_data['ping_status'] ) && 'open' !== $post_data['ping_status'] && 'closed' !== $post_data['ping_status'] ) {
+		if ( 'open' !== $post_data['ping_status'] && 'closed' !== $post_data['ping_status'] ) {
 			unset( $post_data['ping_status'] );
 		}
 
@@ -1681,6 +1681,14 @@ protected function _insert_post( $user, $content_struct ) {
 		 */
 		$post_data = apply_filters( 'xmlrpc_wp_insert_post_data', $post_data, $content_struct );
 
+		// Remove all null values to allow for using the insert/update post default values for those keys instead.
+		$post_data = array_filter(
+			$post_data,
+			static function ( $value ) {
+				return null !== $value;
+			}
+		);
+
 		$post_ID = $update ? wp_update_post( $post_data, true ) : wp_insert_post( $post_data, true );
 		if ( is_wp_error( $post_ID ) ) {
 			return new IXR_Error( 500, $post_ID->get_error_message() );

tests/phpunit/tests/date/xmlrpc.php

@@ -24,16 +24,6 @@ public function tear_down() {
 	 * @covers wp_xmlrpc_server::mw_newPost
 	 */
 	public function test_date_new_post() {
-		if ( PHP_VERSION_ID >= 80100 ) {
-			/*
-			 * For the time being, ignoring PHP 8.1 "null to non-nullable" deprecations coming in
-			 * via hooked in filter functions until a more structural solution to the
-			 * "missing input validation" conundrum has been architected and implemented.
-			 */
-			$this->expectDeprecation();
-			$this->expectDeprecationMessageMatches( '`Passing null to parameter \#[0-9]+ \(\$[^\)]+\) of type [^ ]+ is deprecated`' );
-		}
-
 		$timezone = 'Europe/Helsinki';
 		update_option( 'timezone_string', $timezone );