A traffic stop is an official interaction. What's the source of your belief that you don't have to answer questions about firearms in your vehicle if a police officer in the course of an official interaction asks?
The 5th amendment?
It's a test to assess the driver's attitude and psychology. Even if you answer honestly that you have no guns or that you have guns and they're all legal, the officer has no reason to believe you. The officer doesn't care about guns in the trunk. He or she is looking for tells, nervousness, or is hoping to catch you in a lie. Whether it's guns, drugs, dead bodies, a million dollars, or nuclear weapons in the trunk is secondary to the officer. It's a fishing expedition, and I think it's absurd.