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