The reason you don't see any spots on the bright white image is two-fold:

 - Light is entering your camera at an aperture size equal to the throat diameter of the Nikon F-mount. That is larger than your camera's sensor. The dust spots on your camera's sensor stack are not visible because the light is coming from around the edges at all angles and there are no shadows being projected onto the sensor below the glass cover.
 - The exposure is so bright that the entire image is at maximum saturation/pure white. Any details that might have been visible at a lower exposure level are completely blown out by the overexposure.

Your example is exactly what one would expect with something on your sensor (dust or spots of dried liquid, etc.) when you take a photo with no lens on and the exposure completely blown out.