OGM-618

This avoids storing nulls in association rows for MongoDB, CouchDB and the map-based dialects. Ideally, we'd have some mapping tests for this, but I couldn't get to that for now. It looks good alright from visual inspection. Neo4j does not store null properties, the null values from the query are ignored (checked that).
Interesting, is there ever a case where we'd put null as association row? Why would we do that?