[MERGE FIRST]Fix to the public api for missing elements

If an outcome or question was missing an element
we were blowing up, when it is actually legal to
have them missing. Now return nil instead.
(This was the behaviour before 0.11.0)
A test around this would be good, again, happy for a follow up PR rather than this one.

We should get a fixed version of Smartdown out to unblock application work relying on it.