How to prevent an approval rule exception error when employee has no line manager

Summary:

In exceptional cases, the approval rule fails because approval participant Line Manager does not exist. How to handle this?

Content:

For the Termination (v2 or v3) process we have configured approval by HR and an FYI to the Line Manager and this works well. Occasionally, when the employee has become an ex-employee, HR performs a 'Correct Termination' action. In this situation, the ex-employee no longer has a line manager (standard system behavior) and hence the FYI cannot be sent which causes the approval rule to fail (issue: BPEL_TASK_30082). I'm trying to catch this situation in an IF condition in the approval rule, but nothing seems to work.