[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)
It only broke in https://github.com/alphagov/smartdown/commit/dc39afa6225f5ddf6e48dae4e59339d9b042a686, so quite recently. 

We should definitely have some test coverage here - ideally of a generalised govspeak/markdown builder, as it's in two places - but IMO neither of those are blockers for this PR and i'd be happy for that to get added in a follow up PR.