Returns the parent of the current job if the parent-child relationship is established or
null if the job has no parent or was successfully completed.
Accesses to this property are not idempotent, the property becomes
null as soon as the job is transitioned to its final state, whether it is cancelled or completed, and all job children are completed.
For a coroutine, its corresponding job completes as soon as the coroutine itself and all its children are complete.