Zero’s back in town, and the town is jollier for it. That’s Zero as in Mostel, in the ursine form of Jim Brochu, who has brought his one-man biographical show, Zero Hour, back to the Theatre at St. Clement’s. It won him a Drama Desk Award back in 2010, and in this new incarnation, if anything, the author and star is more formidable, more unpredictable, more voluble—more Zero.