In “The Siege,” New York City is under attack by Arab terrorists, and everyone responds by getting very, very tense. You can see the tension in the way the characters glower and snipe at one another — barking into their mobile phones, meditating over cryptic faxes, acting as if they’d just been caught in the temple-throbbing vortex of an aspirin commercial. For a while, at least, the movie looks like a thoughtful, exciting thriller. There’s just one catch: The director, Edward Zwick, who made “Courage Under Fire,” “Legends of the Fall,” and “Glory,” knows that he’s grabbed on to a provocative, incendiary subject, but he’s afraid of what’s incendiary about it. He wants to get us all riled up about the evils of terrorism and, at the same time, to get us outraged at our own outrage. It’s no wonder the result caves in on itself. In its grimly competent way, “The Siege” isn’t a bad movie, but it’s an empty and joyless one — an oxymoronic political-suspense thriller, a piece of guilty demagoguery.