fix(StoryItemRelation): fails when a set item has no position.

This is caused by an unknown bug which hasnt been resolved yet.
Reposition items are probably creating invalid set items with nil position.
The change here stops the build_items from failing.

lib/supplejack/story_item_relation.rb

@@ -54,9 +54,11 @@ def respond_to_missing?(_method, *_args, &_blocks)
     private
 
     def build_items(items)
-      @items = items.map do |hash|
-        Supplejack::StoryItem.new(hash.merge(story_id: story.id, api_key: story.api_key))
-      end.sort_by(&:position)
+      story_items = items.map do |item|
+        Supplejack::StoryItem.new(item.merge(story_id: story.id, api_key: story.api_key))
+      end
+
+      @items = story_items.sort_by { |i| i.position.to_i }
     end
   end
 end