I think I've invented the term, because I've not heard anything like it elsewhere.
Basically, a null protagonist is a central character whose attributes are wholly defined by the player. I'm not talking about character generation or stats or anything like that - I'm talking about personality, method of operation, appearance, backstory, everything. And by 'defined' I mean 'left up to them to imagine/think about if they want to.'
A couple of examples would help. The closest thing I remember to a true null protagonist in any game I've played is the player character in The Seventh Guest. The player character does exist - despite what you may think - as Stauf talks to him and so does the boy (and through him, to the player). Yet the only things the game defines about that character are its name - Ego, and even that's only in the manual - and the fact that it's inside this house.
A more recent and widespread example is Half-Life's Gordon Freeman. What do we know about him? MIT graduate in theoretical physics; research associate at Black Mesa; male; probably of Scottish descent; handy with a gun. That's really all we know about him - plus his supposed appearance, but that's only shown on the box (well, and in multiplayer). He never speaks, so we don't know what he sounds like, and we don't know what he's thinking. He never acts independently of the player. He's not far from being a null protagonist either.
What's so great about the null protagonist? It represents the minimum that must be provided for the player to exist in the game world - their direct avatar. You can't take anything away because there's nothing left to take; you can't add anything because that takes control of the character definition away from the player.
I wonder what it would have been like if Valve hadn't included an "artist's impression" of Freeman on the box art. What would people think he looked like? How would it have affected the game?