DATAMONGO-987 - Avoid creation of lazy-loading proxies for null-values.

We now avoid creating a lazy-loading proxy if we detect that the property-value in the backing DbObject for a @Lazy(true) annotated field is null.

Original pull request: #207.
DATAMONGO-987 - Some polishing in MappingMongoConverter.

Let getValueInternal(…) use the provided SpELExpressionEvaluator instead of relying on the MongoDbPropertyValueProvider to create a new one. Removed the obsolete constructor in MongoDbPropertyValueProvider.