Oops, fix crashing bug when Item Getting Guide has *no* Dyeworks

Lmao I've been testing with an outfit that has all the kinds of items,
so I didn't notice that this new refactor to `@items[:dyeworks]` style
of tracking the items returns `nil` when there's none, instead of `[]`.
(I always make this mistake when I use `group_by` lmao sob)

In this change, we give the `@items` hash a default value, so that will
stop happening!

app/controllers/items_controller.rb

@@ -117,7 +117,7 @@ class ItemsController < ApplicationController
     item_ids = params[:ids].split(",")
     @all_items = Item.where(id: item_ids).includes(:nc_mall_record).
       includes(:dyeworks_base_item).order(:name).limit(50)
-    @items = @all_items.group_by(&:source)
+    @items = @all_items.group_by(&:source).tap { |i| i.default = [] }
 
     assign_closeted!(@all_items)