survey: Differentiate no answers from not shown questions

[tahini]

Currently, in the responses, a null value can mean either that the question was not shown or that the participant did not answer. We do not have a way to differentiate those 2 cases. And while it is possible to give a default value in the conditional when the question is not shown, it means it's every widget's responsibility to decide what to put. While it may be ok for some (like 'nonApplicable' to driving license for someone < 16 years old), it may not be desirable to put such a burden to every widget. The framework should have a way to make that explicit.