This bug is only relevant with hibernate.create_empty_composites.enabled=true and a collection key that is composite.
When doing a left join fetch with an empty collection, a null collection key is resolved as an empty composite. The collection element ends up being null (because there is no element), and is excluded from the collection. After the query is executed, the PersistentCollection remains uninitialized when it should be initialized as an empty collection.
Fixed in master.