Improve Mongoid::Contextual::Aggregable::Mongo#aggregates

1. Stop Mongoid::Contextual::Aggregable::Mongo#aggregates from checking if collection is empty. It was required when map/reduce was used, but not anymore with Aggregation Framework.


2. 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.