Fix moving closet hanger to null-list

Rails 5 added new validation on `belongs_to` to ensure the corresponding record exists. In the case of moving to the null list, this shouldn't trigger!

I wish we could flag that specifically `nil` is okay, but other values should be validated? But oh well, this is fine!

app/models/closet_hanger.rb

@@ -1,6 +1,6 @@
 class ClosetHanger < ApplicationRecord
   belongs_to :item
-  belongs_to :list, :class_name => 'ClosetList'
+  belongs_to :list, class_name: 'ClosetList', optional: true
   belongs_to :user
 
   delegate :name, to: :item, prefix: true