The Whitney’s “American Effect” show is an idea worth exploring, on its face, writes Blake Gopnik. But there’s a fatal flaw in the working out of the idea. “Many, maybe most, of the dozens of brilliant artists working today aren’t American, and it’s hard to think of a single one of those foreigners whose art, however socially engaged, centers on ideas about America. Of course, that’s why none of those artists is in the Whitney show. The exhibition mostly features little-known foreign artists who deserve their lack of recognition.”