DATAJPA-525 - Added null guard to JpaMetamodelMappingContext.

Added a null guard to JpaMetamodelMappingContext.setInitialEntitySet since some JpaProviders (Hibernate) return null values from ManagedType.getJavaType() for embedded types values. 
This causes trouble with libraries like Hibernate Envers.
Since the returned java types are collected in the initialEntitySet 
we make a filtered copy to make sure that no null entity types are passed to the underlying mapping context.

Original pull request: #89.