Please see attached test files.
In the attached files, A Stuff has multiple Things. Each of the Things has a parental reference to their Stuff.
The test attempts to do an IS EMPTY check in JPQL against the stuff's @OneToMany mappedBy collection of things. This is the resulting error:
Tested against mysql Ver 14.14 Distrib 5.6.38, for Win64 (x86_64)