Bugfix for unmigrated ContentDates in outline generation.

The 2.2.0 release introduced the ability to select a subset of dates by block type, which is useful for grabbing the limited number of blocks necessary to generate a course outline (just course, chapter, and sequential). It was intended to be backwards compatible with unmigrated data, but there was a bug in this fallback behavior that resulted in not pulling back any data at all (and sometimes caching that). This would cause outlines to be generated with null values for these fields, resulting in content being incorrectly hidden from students. This release fixes the issue.