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.

git-svn-id: https://develop.svn.wordpress.org/trunk@47580 602fd350-edb4-49c9-b593-d223f7449a82

src/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;
 	}