Actually, a Calvinist would more likely tell you that Hitler was predestined to be unregenerate and that he is burning in Hell right now. A "deathbed confession" would not happen unless God willed it to happen. So, yes, they pretty much are empty. None of us (Calvinists) believe we can "fool God", and live lives of debauchery and then cash in our redemption card on our deathbeds. God saves whom He chooses to save.
You contradict yourself. You have no way of knowing which are "empty" and which are not.