Please tell us why you think it's riskier than a flight with cabin crew on board.
Because if there is no cabin crew and the passenger cabin is empty, there is no one around to detect fires early or fight them while both flight crew remain in cockpit flying the plane and handling their systems controls.
It is true that with no one in cabin, there is less chance of a fire breaking out to begin with. But does this outweigh and more than outweigh the problems detecting and fighting a fire if and when it does break out?