Export: Update export_wp() to handle get_comment() returning null due to filter.

The `get_comment` filter now explicitly documents returning `null` in addition to a `WP_Comment` object. This allows the filter to be used to exclude comments from an export. The `get_comment()` function already supported returning `null`.

Developed in WordPress/wordpress-develop#8383

Props abcd95, WPExplorer, desrosj, mukesh27, westonruter, SirLouen, lbones, mdibrahimk48, audrasjb, jorbin, wildworks, hellofromTonya, saurabh.dhariwal, mabfahad.
Fixes #61244.

Built from https://develop.svn.wordpress.org/trunk@61369


git-svn-id: https://core.svn.wordpress.org/trunk@60681 1a063a9b-81f0-0310-95a4-ce76da25c4cd

Author: westonruter

wp-admin/includes/export.php

@@ -685,7 +685,12 @@ function wxr_filter_postmeta( $return_me, $meta_key ) {
 	endforeach;
 
 				$_comments = $wpdb->get_results( $wpdb->prepare( "SELECT * FROM $wpdb->comments WHERE comment_post_ID = %d AND comment_approved <> 'spam'", $post->ID ) );
-				$comments  = array_map( 'get_comment', $_comments );
+				$comments  = array_filter(
+					array_map( 'get_comment', $_comments ),
+					static function ( $comment ) {
+						return $comment instanceof WP_Comment;
+					}
+				);
 				foreach ( $comments as $c ) :
 					?>
 		<wp:comment>

wp-includes/comment.php

@@ -240,9 +240,12 @@ function get_comment( $comment = null, $output = OBJECT ) {
 	 *
 	 * @since 2.3.0
 	 *
-	 * @param WP_Comment $_comment Comment data.
+	 * @param WP_Comment|null $_comment Comment data.
 	 */
 	$_comment = apply_filters( 'get_comment', $_comment );
+	if ( ! ( $_comment instanceof WP_Comment ) ) {
+		return null;
+	}
 
 	if ( OBJECT === $output ) {
 		return $_comment;

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '7.0-alpha-61368';
+$wp_version = '7.0-alpha-61369';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.