SPOILER ALERT! Though it's hard to see where more nihilism could be squeezed into Mike Nichols' adaptation of Closer, a closer (sorry!) look at the Patrick Marber play on which the film is based reveals that Natalie Portman's Alice originally had a darker fate: In the play, she gets mowed down by a car on Madison Avenue (the only lasting damage from an earlier bump by a taxi was her introduction to Jude Law's Dan). ''The film ends one scene before the play ends,'' says Marber of the tweak. ''I'm perfectly happy with it.'' Happy? What is this happy of which you speak?