Incorrect representation of empty prereqs in Master

[AbdallahS]

The representation of Prereqs in a Course object is a list of list of courses in DNF (a disjunction of conjunction of other courses). If a course has no actual prerequisite courses, then we should represent this with the list [[]] (a disjunction of an empty conjunction). That way, the empty conjunction is trivially true, so the disjunction holds and we can see that the prereqs for the course are satisfied.

In Master, this is currently represented as [] (instead of [[]]) so trivially false instead of trivially true. Not sure if it's been addressed in the banches, sorry.