Approval chain with Project Manager

Content

Good day, 

Could you please help us with Transfer approvals.

We have a following case:

• Transfer of Employee can be initiated by Line Manager or Human Resource Specialist with different approval flows  (this part is not the problem and we can realize it)
• Project Manager should be included in approval chain. But the problem is that some employees have Project Manager and another don’t have Project Manager (so this field is blank). How we can make a check of this information and create 2 rules to avoid workflow issues?
  We've tried the following rule but it doesn't work:
  (GetManager("PROJECT_MANAGER",Task.payload.Worker's Proposed Assignment.result.Assignment Supervisor) == null)