MongoStore: Document or List mapping change cause NPE in clearDirty()

I've found an issue in current implementation of MongoStore on Document or List mapping type handling.

If your schema contains a field like this :

{ "name": "keywords",
   "type": { "type": "array", "items": "string"},
   "default": null
}

If you choose to latter defaults to an empty array instead of null value like this

{ "name": "keywords",
   "type": { "type": "array", "items": "string"},
   "default": []
}

loading old data will fail because of clearDirty method will try to also clear a null object.

persistent.clearDirty();

I implemented and tested a fix which always return a DirtyListWrapper or DirtyMapWrapper instead of previous null value.