Build/Test Tools: Avoid an empty "title" heading if no title is set in...

Build/Test Tools: Avoid an empty "title" heading if no title is set in `_wp_die_handler_exit()` and `_wp_die_handler_txt()`.

Fixes #50297.
Built from https://develop.svn.wordpress.org/trunk@47882


git-svn-id: https://core.svn.wordpress.org/trunk@47656 1a063a9b-81f0-0310-95a4-ce76da25c4cd