MySQL Composite PK with Nullable FKs

First off, let me preface this question by stating that I'm really a pretty terrible data modeler. I know only enough to be dangerous.

The table I'm building has four foreign keys, two of which reference the same table. Here's the create statement for that table.

CREATE  TABLE IF NOT EXISTS `abnr`.`reputation_event_log` (  `id` INT NOT NULL AUTO_INCREMENT ,  `reputation_event_id` INT NULL ,  `giver_user_id` INT NULL ,  `receiver_user_id` INT NULL ,  `review_id` INT NULL ,  `giver_point_value` SMALLINT NULL DEFAULT 0 ,  `receiver_point_value` SMALLINT NULL DEFAULT 0 ,  `created_at` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ,  PRIMARY KEY (`id`) ,  INDEX `fk_reputation_log_user` (`giver_user_id` ASC) ,  INDEX `fk_reputation_log_user1` (`receiver_user_id` ASC) ,  INDEX `fk_reputation_log_review` (`review_id` ASC) ,  INDEX `fk_reputation_log_reputation_event` (`reputation_event_id` ASC) ,  CONSTRAINT `fk_reputation_log_user`    FOREIGN KEY (`giver_user_id` )    REFERENCES `abnr`.`user` (`id` )    ON DELETE NO ACTION    ON UPDATE NO ACTION,  CONSTRAINT `fk_reputation_log_user1`    FOREIGN KEY (`receiver_user_id` )    REFERENCES `abnr`.`user` (`id` )    ON DELETE NO ACTION    ON UPDATE NO ACTION,  CONSTRAINT `fk_reputation_log_review`    FOREIGN KEY (`review_id` )    REFERENCES `abnr`.`review` (`id` )    ON DELETE NO ACTION    ON UPDATE NO ACTION,  CONSTRAINT `fk_reputation_log_reputation_event`    FOREIGN KEY (`reputation_event_id` )    REFERENCES `abnr`.`reputation_event` (`id` )    ON DELETE NO ACTION    ON UPDATE NO ACTION)ENGINE = InnoDBDEFAULT CHARACTER SET = utf8COLLATE = utf8_general_ci;

The indexes I'm concerned with for this post are fk_reputation_log_user and fk_reputation_log_user1. Every reputation event has a giver, but only some have a receiver. I would like this FK to be nullable, but I don't know how to do that, or if it's even "allowed".

I also toyed with the idea of making all FK columns part of the primary key to have database-level protection against duplicate log entries - but that won't work since PK columns have to be NOT NULL.

If you need more detail, please state that in the comments. Thanks!

(Yes, this is for a reputation system not too dissimilar from what SO has)