The null car is a placeholder vehicle that is shown to players racing against a DLC car they do not own. Usually the easiest way to solve this is by downloading the free car from every available car pack. Unfortunately, Forza 4's Porsche Expansion Pack did not include a free car, and the only way to avoid seeing the Null Car is by buying the pack. The Null Car in Forza 4 is a blacked-out Volkswagen Jetta with no visible interior model.
On occasion, hackers/modders tend to manipulate the game to have this car in their garage.