Edit some Position fields only if no incumbent

Hello Experts

We are facing an issue here.

We use Position Sync and therefore data goes to the employee, once a BP changed the Job and it changed historically into payroll which caused a great problem.

I found no option to block the field if position is occupied. Because if not occupied then users could indeed change the job, for example the requisition has been reviewed and a SR req turns JR then the position is changed and we are all clear.

The Expression language in doc ID does not work: Fusion Global HR: Need to Identify If a Position Has No Incumbent So Business Unit Can Be Modified (Doc ID 2858301.1)