NEW YORK — It was supposed to be a great holiday shopping season. Many investors had expected department stores to enjoy robust sales in light of a U.S. economy buoyed by low unemployment, higher wages, strong consumer confidence and cheap gas. So when Macy’s and Kohl’s reported lackluster numbers on Thursday, they were taken aback, sending retail stocks into a tailspin and calling into question whether such mall-based chains can compete in a changing landscape where shoppers are shifting more of their spending online. Macy’s saw only a slight increase of 1.1 percent in sales during November-December at stores opened at least a year.