Fix or MAGN-4059 with hardening for release builds and a data request trap for debug

I can't repro the case that is causing this nested data to be empty, so I've added a trap that detects it and requests more data on debug builds, and on release builds hardens the behaviour
I thought about doing that and rejected it.

There is a separation between "it has nested structure" and "it has nested data". I would rather find out why this case is ever occurring and squash it. That's easier to do with the test here, then adjusting the accessor test which I think will result in us hiding the issue more....