Perhaps no significantly bright stars. I find it unlikely that there are zero stars in that field of view, especially since the probe and planet are most likely roughly in the plane of the galaxy.
For one thing, it assumes that only a small fraction of the image sensor's surface is actually capable of detecting light, which would make it very inefficient. Typical consumer digital cameras can capture over 30% of the incoming energy, and presumably New Horizons' camera can do better, being specifically designed for a low-light environment.
More importantly, no atmospheric "diffusion" is required to spread out the incoming light, because it's occurring anyway through diffraction at the camera's aperture. The angular resolution of the LORRI camera is specified as about 1 arcsecond, which is comparable to the atmospheric seeing conditions on a relatively clear night on Earth.