In recent years, Michael Bay has focused nearly all his directorial energy on the “Transformers” franchise, reinforcing his reputation as the go-to guy for effects-driven spectacles. Bay may not be Hollywood’s best storyteller, but he knows how to blow things up, and he has an uncanny knack for seamlessly blending practical footage with breathtaking digital imagery. These skills are put to good use in “Transformers: Age of Extinction,” the fourth film in a series that’s made oodles of money despite an astonishing lack of imagination.