Handle case where first_published_at is nil
Handle case where first_published_at is nil

Some imported announcements won't have a first_published_at date
included in the CSV file, but we still need to allow the document to be
associated with ministers.

Either way, we definitely shouldn't crash.

robinwhittleton/whitehall,hotvulcan/whitehall,hotvulcan/whitehall,askl56/whitehall,alphagov/whitehall,ggoral/whitehall,hotvulcan/whitehall,YOTOV-LIMITED/whitehall,YOTOV-LIMITED/whitehall,robinwhittleton/whitehall,YOTOV-LIMITED/whitehall,robinwhittleton/whitehall,ggoral/whitehall,ggoral/whitehall,askl56/whitehall,askl56/whitehall,alphagov/whitehall,alphagov/whitehall,alphagov/whitehall,YOTOV-LIMITED/whitehall,ggoral/whitehall,robinwhittleton/whitehall,hotvulcan/whitehall,askl56/whitehall