XNY/Star Max/GC Images/Getty ImagesThe unofficial US Open dress code may call for tennis whites and preppy polos, but that won’t stop Kaia Gerber from showing up in a little black dress. Still, the model-turned-actor has a way of making her signature silhouette tennis-appropriate. On Tuesday evening, Gerber attended the 2026 US Open exhibition honoring legend Roger Federer in New York City. At first glance, she appeared to stick to a familiar ou…