OMG-587 Errors with embedded list

https://hibernate.atlassian.net/browse/OGM-587

I've moved the test case in core and it showed failure in Neo4j, CouchDB and MongoDB.
Hum, but the original issue says:

> As soon as I try to save the user with an empty embedded document, the process fails with "com.mongodb.MongoException$DuplicateKey error" although the entity has not been stored so far.

But in the test we don't store an empty embedded document?