Tracked this down, it was caused by HHH-4848 Closed on Wed Feb 3 06:58:46 2010 +0000
see: https://hibernate.atlassian.net/browse/HHH-4848

The check used to skip null values with a continue, but was changed to throw an exception. As such HHH-10956 Open is a regression caused by HHH-4848 Closed

This is the (relevant) commit log for the offending file

commit 3906816ee2625987796feb587ca374a55c3374ae
Author: Steve Ebersole steve@hibernate.org
Date: Mon Dec 19 16:35:48 2016 -0600

HHH-11274 - EntityManagerFactoryImpl.getIdentifier uses deprecated version of getIdentifier

commit 87e3f0fd28d7229b0a35032292ff32f542c22010
Author: Steve Ebersole steve@hibernate.org
Date: Tue Apr 19 13:14:02 2016 -0500

HHH-10664 - Prep 6.0 feature branch - merge hibernate-entitymanager into hibernate-core (first sucessful full compile of consolidated hibernate-core)

commit 611f8a0e1c3764dc4505a32294ff0fbeaa8d2e6b
Author: Steve Ebersole steve@hibernate.org
Date: Mon May 18 13:35:26 2015 -0500

HHH-9803 - Checkstyle fix ups

commit 8fe5460ec03942e974d4aae7d9060e20bab7f990
Author: Steve Ebersole steve@hibernate.org
Date: Sun Nov 24 20:07:37 2013 -0600

HHH-8741 - More checkstyle cleanups

commit 19791a6c7d17f6f5d74dd9b42bdb4fcb7f5a9be1
Author: Steve Ebersole steve@hibernate.org
Date: Fri Mar 18 15:51:43 2011 -0500

HHH-6026 - Migrate bytecode provider integrations to api/spi/internal split

commit 839e23458aa0c50e1dc0d8f1a1bed87dbff49f3b
Author: Steve Ebersole steve@hibernate.org
Date: Mon Feb 8 19:27:00 2010 +0000

HHH-4704 - Pass session into EntityTuplizer#setIdentifier

git-svn-id: https://svn.jboss.org/repos/hibernate/core/trunk@18733 1b8cb986-b30d-0410-93ca-fae66ebed9b2

commit 53b1890c1b2b43a8dc89f0f09d6d3b6b6f3cbdc1
Author: Steve Ebersole steve@hibernate.org
Date: Fri Feb 5 02:42:55 2010 +0000

HHH-4848 - Derived identities: Derived entities using @IdClass and mapping a @XToOne are not supported

git-svn-id: https://svn.jboss.org/repos/hibernate/core/trunk@18698 1b8cb986-b30d-0410-93ca-fae66ebed9b2