This issue was introduced by a side-effect in 2.1.3 with
d9f9228d95c5ab2a200b3e2d2b358c598267afb1. In this commit read-only
objects where prevented to be updated. This lead to an invalid check not
being performed in UnitOfWork#computeChangeSet which was present before
where an association that was null would be injected into the
originalEntityData using the UnitOfWork#setOriginalEntityProperty()
method in the AbstractHydrator.
This commit now explicitly sets this field to null using the same API so
that is present during UnitOfWork#computeChangeSet.
bf32125bad