HHH-4773 @CollectionId does not force the id column to not-null

@CollectionId describes an identifier column for a bag (ie an idbag).
but it does not force the id to be not null.

this can be reproduced by org.hibernate.test.annotations.collectionelement.indexedCollection.IndexedCollectionOfElementsTest on DB2 (because of DB2 has more restrict validation seems, it does not allow nullable column to be primary key)

http://opensource.atlassian.com/projects/hibernate/browse/HHH-4773

and also, this will revert the change made in JBPAPP-3375