Some REST kogito API expects processInstanceId as parameter. When a not existing processInstanceId is specified, a 204 http error code is returned (rather than 404, which is the proper one for a not found issue) 

Reason is that generated code is returning null rather than throwing ProcessInstanceNotFoundException or WorkItemHandlerNotFoundException. 

Besides modifying code generation to throw these exceptions, we need to ensure proper mappers are configured for JAX/RS (Quarkus) and Spring MVC (Spring boot)  and that test that are not expecting a 204 are changed accordingly.