Ballet lovers (and dancers) know it well: the Nutcracker threshold, that moment in early January when another visit to the Land of Sweets might kill you. But New Yorkers are lucky: They have New York City Ballet's winter season to revive them from the sugar crash. You could sense the collective relief in the David H. Koch Theater on Tuesday, at the company's season-opening performance, a program of four ballets by George Balanchine.