Display relevant message when no signature set on preview.

Added a text string when there is an empty string on preview of new or current signature on profile modify page. Also removed some code that looked to be not used in `Profile.template.php` as the preview is being handled by xml.php, if I'm wrong on this assumption let me know I can revert the change in `Profile.template.php`.

Signed-off-by: ziycon <ziycon.eire@gmail.com>