Hibernate51CompatibilityTransformer#transform should always return null if no class change is made

Currently, Hibernate51CompatibilityTransformer#transform is not always returning null when no changes are made to the application class, which could slow application deployment. This only happens for Hibernate ORM native applications that extend/implement certain ORM classes but do not require class transforming.

We did not measure the performance impact, so not sure how important this change is.