The PDP shall return INDETERMINATE when the environment attributes are missing.
I suggest to add the possibility that this behavior can be turned off. But with a clear WARNING that this may lead to unexpected behavior because the PDP runs in a way that is off the specification.
Another possibility (for testing or something) could be that the value could also be SET_WHEN_MISSING. This means that the PDP sets the environment attributes automatically when a request enters the PDP and it has missing environment attributes.