As No Doubt ticked towards the end of the first night of their Las Vegas Sphere residency on Wednesday with “Just a Girl,” a period-perfect hit that’s become a generational touchstone over the past few decades, lead singer Gwen Stefani put a nostalgic cap on the evening. “I wrote this song out of pure innocence in a time where I was just becoming aware of myself and my surroundings,” she told the 20,000-odd fans in the audience. “I always thought I would grow up too much to sing it and it would be out of style. But you tell me if it’s still relevant or stylish. It’s up to you guys.” The response to such an era-defining hit was, of course, rapturous. But No Doubt’s inaugural Sphere performance wasn’t about picking the low-hanging fruit of a greatest hits show, or trying to reclaim past glory.…