That section in my ACI copy, 318-14, just says "Value of Vn accross the assumed shear plane shall be calculated in accordance with 22.9.4.2 or 22.9.4.3. Vn shall not exceed the value calculated in accordance with 22.9.4.4"
It mentions minimum shear-friction reinforcement areas, but doesn't say anything about shear across unreinforced areas. I suppose this value is normally ignored. In my case, I have a slab at the bottom of a hole with concrete walls. I would like to count on some unreinforced friction to resist the possibility of small uplift forces.