As I said, EXPIRED sounds good for if you are inferring it will be used when this event is raised due to an effect timing out.  But what about using milk?  I almost think a TREATED will be needed also.

I still don't know what you mean by null being used.  Are you just recognizing that someone could call the event with a null cause?  That's fine.  But I don't see why a CraftBukkit implementation would do that.