It was hard to know what to expect from “Guava Island.” Last year, Donald Glover and Rihanna filmed the mysterious project with director Hiro Murai (who’s also directed multiple episodes of “Atlanta” and the music video for “This is America”, then they said almost nothing about it until debuting the film at Coachella and releasing it on Amazon. “Guava Island” turns out to be a 54-minute, fable-like story of a musician named Deni (Glover) and his girlfriend Kofi (Rihanna) on a fictional Caribbean island.