Make Mongoid::Contextual::Aggregable::Mongo#aggregates return a better hash

Hash has count set to a number of documents with the provided field. When
there're no documents with that field, then count is 0 and other keys are set to
nil.

Previous behavior was inconsistent:
- when there were any documents with the requested field, count was set to a
  number of documents with that field
- otherwise, count was set to a number of documents matching the query,
  but not to a numer of documents with the requested field, which was 0

Makes Mongoid::Contextual::Aggregable::Memory behave in a similar way.