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

The `wp_xmlrpc_server::mw_newPost()` method creates a new post via `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.

For this commit, I've:
* Ensured that all arguments are defined before they are `compact()`-ed together to the arguments array.
* Verified that the default/fall-back value of the arguments as set within the `wp_xmlrpc_server::mw_newPost()` method are the same as the default/fall-back values used in the `wp_insert_post()` function.
* Verified that arguments which do not have a default/fall-back value defined in the `wp_insert_post()` function are handled correctly.
    - This was not the case for `$post_name`, which would previously already get an empty string default value in the `wp_xmlrpc_server::mw_newPost()` function, but then in the `wp_insert_post()` function, this would prevent the slug generation from being activated.
        Fixed now by setting the default in the `wp_xmlrpc_server::mw_newPost()` function to `null`..
    - The `page_template` argument was handled, but not documented in the `wp_insert_post()` function. I've now documented the argument in the `wp_insert_post()` function docblock.
        Note: there are more than likely several other potential arguments missing from that list, but verifying the whole list is outside the scope of this particular commit.

Includes minor simplifications, such as:
* Setting a default ahead of an `if`, instead of in an `else` clause (as long as no function call is needed to set the default).
* Remove the unnecessary logic duplication in the `$post_status` switch.
* Using a combined concatenation + assignment operator for adding `$post_more`.

Fixes various errors along the lines of:
```
1) Tests_XMLRPC_mw_editPost::test_draft_not_prematurely_published
strpos(): Passing null to parameter #1 ($haystack) of type string is deprecated

/var/www/src/wp-includes/formatting.php:2497
/var/www/src/wp-includes/class-wp-hook.php:308
/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:5616
/var/www/tests/phpunit/tests/xmlrpc/mw/editPost.php:315
```
```
23) Tests_XMLRPC_mw_editPost::test_draft_not_prematurely_published
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:5615
/var/www/tests/phpunit/tests/xmlrpc/mw/editPost.php:315
/var/www/vendor/bin/phpunit:123
```

Author: jrfnl

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

@@ -5399,30 +5399,27 @@ public function mw_newPost( $args ) {
 
 		// Let WordPress generate the 'post_name' (slug) unless
 		// one has been provided.
-		$post_name = '';
+		$post_name = null;
 		if ( isset( $content_struct['wp_slug'] ) ) {
 			$post_name = $content_struct['wp_slug'];
 		}
 
 		// Only use a password if one was given.
+		$post_password = '';
 		if ( isset( $content_struct['wp_password'] ) ) {
 			$post_password = $content_struct['wp_password'];
-		} else {
-			$post_password = '';
 		}
 
 		// Only set a post parent if one was given.
+		$post_parent = 0;
 		if ( isset( $content_struct['wp_page_parent_id'] ) ) {
 			$post_parent = $content_struct['wp_page_parent_id'];
-		} else {
-			$post_parent = 0;
 		}
 
 		// Only set the 'menu_order' if it was given.
+		$menu_order = 0;
 		if ( isset( $content_struct['wp_page_order'] ) ) {
 			$menu_order = $content_struct['wp_page_order'];
-		} else {
-			$menu_order = 0;
 		}
 
 		$post_author = $user->ID;
@@ -5450,8 +5447,8 @@ public function mw_newPost( $args ) {
 			$post_author = $content_struct['wp_author_id'];
 		}
 
-		$post_title   = isset( $content_struct['title'] ) ? $content_struct['title'] : null;
-		$post_content = isset( $content_struct['description'] ) ? $content_struct['description'] : null;
+		$post_title   = isset( $content_struct['title'] ) ? $content_struct['title'] : '';
+		$post_content = isset( $content_struct['description'] ) ? $content_struct['description'] : '';
 
 		$post_status = $publish ? 'publish' : 'draft';
 
@@ -5464,15 +5461,15 @@ public function mw_newPost( $args ) {
 					$post_status = $content_struct[ "{$post_type}_status" ];
 					break;
 				default:
-					$post_status = $publish ? 'publish' : 'draft';
+					// Deliberably left empty.
 					break;
 			}
 		}
 
-		$post_excerpt = isset( $content_struct['mt_excerpt'] ) ? $content_struct['mt_excerpt'] : null;
-		$post_more    = isset( $content_struct['mt_text_more'] ) ? $content_struct['mt_text_more'] : null;
+		$post_excerpt = isset( $content_struct['mt_excerpt'] ) ? $content_struct['mt_excerpt'] : '';
+		$post_more    = isset( $content_struct['mt_text_more'] ) ? $content_struct['mt_text_more'] : '';
 
-		$tags_input = isset( $content_struct['mt_keywords'] ) ? $content_struct['mt_keywords'] : null;
+		$tags_input = isset( $content_struct['mt_keywords'] ) ? $content_struct['mt_keywords'] : array();
 
 		if ( isset( $content_struct['mt_allow_comments'] ) ) {
 			if ( ! is_numeric( $content_struct['mt_allow_comments'] ) ) {
@@ -5536,10 +5533,10 @@ public function mw_newPost( $args ) {
 		}
 
 		if ( $post_more ) {
-			$post_content = $post_content . '<!--more-->' . $post_more;
+			$post_content .= '<!--more-->' . $post_more;
 		}
 
-		$to_ping = null;
+		$to_ping = '';
 		if ( isset( $content_struct['mt_tb_ping_urls'] ) ) {
 			$to_ping = $content_struct['mt_tb_ping_urls'];
 			if ( is_array( $to_ping ) ) {
@@ -5555,12 +5552,11 @@ public function mw_newPost( $args ) {
 			$dateCreated = $content_struct['dateCreated']->getIso();
 		}
 
+		$post_date     = '';
+		$post_date_gmt = '';
 		if ( ! empty( $dateCreated ) ) {
 			$post_date     = iso8601_to_datetime( $dateCreated );
 			$post_date_gmt = iso8601_to_datetime( $dateCreated, 'gmt' );
-		} else {
-			$post_date     = '';
-			$post_date_gmt = '';
 		}
 
 		$post_category = array();

src/wp-includes/post.php

@@ -4024,6 +4024,7 @@ function wp_get_recent_posts( $args = array(), $output = ARRAY_A ) {
  *                                         child terms can have the same names with different parent terms,
  *                                         so the only way to connect them is using ID. Default empty.
  *     @type array  $meta_input            Array of post meta values keyed by their post meta key. Default empty.
+ *     @type string $page_template         Page template to use.
  * }
  * @param bool  $wp_error         Optional. Whether to return a WP_Error on failure. Default false.
  * @param bool  $fire_after_hooks Optional. Whether to fire the after insert hooks. Default true.