How to reproduce: Give yourself a renamed sword, use it to hit another player off a ledge so that he will die from fall damage. The death message will read "null" instead of your name. Alternatively, give a zombie/skeleton a renamed weapon and have it attack you, then jump off to certain death by fall damage. Again, the death message will read "null" instead of the mob's name:
player was doomed to fall by null using itemName
Note: This issue does not occur when the item is not named and therefore will not be printed out.