Run BackfillColumnForBroadcasts only if the column still exists (#6508)

lib/data_update_scripts/20200226193303_backfill_column_for_broadcasts.rb

@@ -1,6 +1,8 @@
 module DataUpdateScripts
   class BackfillColumnForBroadcasts
     def run
+      return unless Broadcast.column_names.include?("sent")
+
       Broadcast.find_each { |broadcast| broadcast.update!(active: broadcast.sent) }
     end
   end