Downgrade most item validations to be *not nil*, rather than present

Been running into the item "Hanging Plushie Grundo Background Item" not
being modelable, because TNT seem to have left its description blank!

Let's be less picky about what data we take in, but keep the intention
of these validations: to ensure that *we* don't make a mistake and
forget a field when importing items!

app/models/item.rb

@@ -23,8 +23,13 @@ class Item < ApplicationRecord
   has_many :dyeworks_variants, class_name: "Item",
     inverse_of: :dyeworks_base_item
 
-  validates_presence_of :name, :description, :thumbnail_url, :rarity, :price,
-    :zones_restrict
+  # We require a name field. A number of other fields must be *specified*: they
+  # can't be nil, to help ensure we aren't forgetting any fields when importing
+  # items. But sometimes they happen to be blank (e.g. when TNT leaves an item
+  # description empty, oops), in which case we want to accept that reality!
+  validates_presence_of :name
+  validates :description, :thumbnail_url, :rarity, :price, :zones_restrict,
+    exclusion: {in: [nil], message: "must be specified"}
 
   attr_writer :current_body_id, :owned, :wanted