NPE in Policy is producing mule to hang

Follow up of SE-13586. In MULE-17670 we fixed the symptom (i.e the NPE), but the runtime shouldn't hang because of an NPE.

From heap dump we checked that threads are alive, and waiting to process.

The most probable root cause of the issue could be an onErrorContinue missing in some point of the policy flux chain (so, the flux is being cancelled, which can explain all symptoms).