Handle querying of objects that have no property for a field that is null

[andyjefferson]

Current SVN trunk will persist any null field by just omitting the property. This matches Neo4j advice about how to handle null property values.

This will obviously cause problems with any query that makes explicit mention of the field, and will get an exception about Node xxx not having the property. We should update any query that is comparing against a particular value to check if the property exists and if comparing with null to check if it doesn't exist.