Hi Gonzalo,
Because the ${task.assignee} maybe not the only one reciepent of notification mail. Maybe someone custom his own mail-template. and make more recipients for the notification mail. So we cannot skip mail sending just when the task assignee is null.
I know my patch is not good enough, but at least it provide a more clearly message to people who want to use this feature to send notfication mail.
Maybe you could give us more information about it. Thank you very much.