When the serialized object is NSNull return nil
Return the NSNull value and let the code handle it

This will put it through the rest of the process, which will eventually turn it into nil.

In NSManagedObject (DCTManagedObjectSerialization) dct_deserializeProperty:withDeserializer: the value will get through the check for nil, causing the deserialization to set the attribute to nil regardless of the setting for shouldDeserializeNilValuesForEntity.