Fix coverage calculation for empty patient populations

#### Story ####
- https://www.pivotaltracker.com/story/show/69113542

#### Notes ####
- if a patient population has "None" or no logic elements, the coverage calculation does not compute up to 100% because the population in question does not have a <code>key</code> attribute
- verified with CMS52v2, where a patient can meet all logic element criteria and not achieve 100% without this fix