[DDC-2996] Fix bug in UnitOfWork#recomputeSingleEntityChangeSet

When calling UnitOfWork#recomputeSingleEntityChangeSet on an entity
that didn't have a changeset before, the computation was ignored.
This method however is suggested to be used in "onFlush" and "preFlush"
events in the documentation.

Also fix a bug where recomputeSingleEntityChangeSet was used
before calculating a real changeset for an object.
Merge branch 'DDC-2996' into 2.4