The tagline at the end of Volvo’s new ad, “Moments,” says: Sometimes the moments that never happen matter the most. If there’s a corresponding idea in filmmaking, it might be this: Sometimes what you don’t show is the most affecting. In “Moments” — an ad as heartwarming/heart-wrenching as any we’ve ever seen — a young girl speculates about the rest of her life before starting her first day of school. What friends will she make? Where will she travel? Who will she meet? What unfolds is a fantasy within a fiction: an entire life in less than four minutes. What you see is beautiful, striking even. But ultimately, it’s just a stencil for what you don’t see. The story — the girl’s life — is the negative space.