I made a mistake when cherry-picking things into #1534. Fixing it also
prompted me to regenerate message things without thinking even though it
wasn't needed here but it helped me catch an issue with a bad shim. I
must not have saved it properly on my computer and missed it before
review.
List types should never be optional if sent to NT because an empty list conveys the same
thing.
The equivalent C++ struct takes the same approach with empty vectors rather than an optional vector.
