"Bridgerton,” the new series based on Julia Quinn’s romance novels and developed by hitmaker Shonda Rhimes and showrunner Chris Van Dusen for Netflix, is a Christmas cookie of a show — a sugary, sparkly treat that serves as a comfort in a grim winter. And in integrating the predominantly White world of romance novels by making some of Quinn’s characters Black, the show inadvertently provides a useful lesson for an entertainment industry struggling to meet viewers’ demands and corporate mandates for more diverse programming.