How to blow up in `access_policy` if `security_context` does not have specified member?

I'm building an `access_policy` approach as per:

https://github.com/cube-js/cube/blob/b8afc8466fabcd50e95aad595675aa505ff48293/packages/cubejs-testing/birdbox-fixtures/rbac/model/cubes/users.yaml#L23-L26

It's a great feature and will massively simplify our config because different tables have different RBAC from the token. However (and I'd imagine it's quite a common case - hence raising it here), we need to blow up if the given `security_context` does not have the specified attribute. If it's a nested attribute then this will blow up correctly (as dereferencing an undefined object), however if it's just `security_context.xxx` it will interpolate as `undefined`.