We discovered a subsection that contained a unit without any content
inside, but because of our logic requiring children, it would never be
marked complete (meaning the subsection, section, and course could thus
never be marked complete). This fixes that by removing the children
check from setting completion, but first gating that code path on the
xblock being an aggregator (to prevent leaves from marking as true
simply because there are no children).
Test fixes include adding a test for the empty aggregator case as
well as some changes to not have an entire course marked complete
because they are all empty aggregators.
6f773d1eca