Mail: Make the check for empty post title in `wp-mail.php` more resilient.

This addresses a regression in [47054], which caused posts sent via email to published with an empty title.

Props whyisjake, Otto42, MarkRH, MattyRob, mukesh27, afragen, pikamander2.
Fixes #49853.
Built from https://develop.svn.wordpress.org/trunk@47580


git-svn-id: http://core.svn.wordpress.org/trunk@47355 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.5-alpha-47578';
+$wp_version = '5.5-alpha-47580';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.

wp-mail.php

@@ -213,7 +213,7 @@ for ( $i = 1; $i <= $count; $i++ ) {
 
 	$post_title = xmlrpc_getposttitle( $content );
 
-	if ( '' === $post_title ) {
+	if ( '' === trim( $post_title ) ) {
 		$post_title = $subject;
 	}