I don't have any 1st person experience with this (or work at Waymo) but there are probably several easy ways they avoid this. The car will probably never drive

I don't have any 1st person experience with this (or work at Waymo) but there are probably several easy ways they avoid this. The car will probably never drive with the door open and all modern cars have sensors to detect this. Second, their cars (and any competent self-driving car) needs to have a very accurate awareness of its surroundings. True it might be difficult to get a clear view of a person just barely outside the car but I'd bet that they have cameras/radar/lidar positioned to see if at least something is there. Lastly, they have internal passenger cameras that can detect whether a person is inside the car.