Consider allowing empty specification in aggregation projection

Currently, we allow an empty projection specification in find()-style projections, which defaults to an exclusion projection, but we ban in it aggregation. As part of projection unification project we may want to lift this restriction and allow an empty projection specification in aggregation as well. That is, the following projection will become valid and will be treated as an exclusion projection with all fields included into the output document:

{$project: {}}