Past editions have featured such things as Santa brandishing a pistol at a burglar about to break a window, Santa with a gift sack stuffed with revolvers and Santa aloft in his sleigh blazing away at planes bearing Soviet markings. By comparison, this year's version is downright modest: A beaming Bernhard H. Goetz, unarmed, sitting astride Santa's knee and receiving a ''full pardon'' for Christmas, with the Manhattan District Attorney, Robert M. Morgenthau, downcast, sulking alongside.