If you sing “Happy Birthday” around a “Glee” star, just make sure you’re in tune, people. The producers of Tribeca Film Festival movie “Struck by Lightning” wanted to surprise cast member Robbie Amell before the premiere Saturday. But when the actors joined in to sing “Happy Birthday,” the film’s star and screenwriter, Chris Colfer, couldn’t help coaching his cohorts by interjecting the lyrics, “Find your key-yyyy,” mid-song. “The room erupted in laughter,” said a spy.