sender_cc must be optional

Author: jnorell

interface/web/mail/form/mail_user.tform.php

@@ -200,6 +200,7 @@
 					'type' => 'TOLOWER')
 			),
 			'validators' => array (  0 => array ( 'type' => 'ISEMAIL',
+					'allowempty' => 'y',
 					'errmsg'=> 'sender_cc_error_isemail'),
 			),
 			'default' => '',