I believe this is actually a Hibernate bug and that the mapping should not be changed.

In 4.3, the key column is generated as non-nullable by default. I'm not sure when this change in default changed.

This seems related to https://hibernate.atlassian.net/browse/HHH-10045?focusedCommentId=70557&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-70557