parentProcessInstanceId in ProcessEventListener is 0

When obtaining parent process id inside ProcessEventListener, for example:

public void afterProcessStarted(ProcessStartedEvent arg0) {

System.out.println("[listener] parent id: " + arg0.getProcessInstance().getParentProcessInstanceId());
	}

The returned value is 0 (in case our process does not have parent)

However, when obtaining parent process instance id over REST API, the result is -1

This discrepancy is caused because when using REST API, the value is taken from ProcessInstanceLog where its explicitly set to -1 in case of missing parent.

When using process event listener, the value seems to be taken from ProcessInstanceImpl where its not initialised in case of no parent, i.e.:

    private long parentProcessInstanceId;

and the setter method is simply not called.

Customer would like to have these two values unified - i.e. -1 value in both - REST and listener.

I think this could be simply achieved by initialising the parent process instance id to default value -1, i.e.:

    private long parentProcessInstanceId = -1;

And if there is some parent, it would be of course overridden by calling appropriate setter.