Ben Stiller has been near the center of American culture for nearly 40 years, from “The Ben Stiller Show” in the early ’90s to “Reality Bites” to “Zoolander” to “Meet the Parents” to “Tropic Thunder” to his work behind the scenes of the Apple TV+ smash “Severance.” But that’s all showbiz stuff; none of us have ever really known the man himself. To see the real Ben Stiller, the human being beneath it all, you have to watch a Knicks game.