Handle newly-released PB items in Item Getting Guide

Oh right, it's possible for `Item#pb?` to return true, but
`Item#pb_color` to return `nil`, if the item has the paintbrush item
description but we can't find a color whose name matches the item name.
This would be expected if a new color were added to Neopets, and PB
items for it were modeled by the community, but we hadn't manually
added the color to the database yet.

Previously, the Item Getting Guide would crash in this scenario. Now,
it correctly handles the possibility of a `nil` value for `pb_color`,
and shows some placeholder info.

To test this, I temporarily edited some item names to not contain the
color name anymore (e.g. "P-rate Elephante Shirt and Vest"), then
loaded the guide and made changes until it no longer crashed.

app/controllers/items_controller.rb

@@ -134,10 +134,10 @@ class ItemsController < ApplicationController
     # Also, PB items have some special handling: we group them by color, then
     # load example pet types for the colors that don't have paint brushes.
     @pb_items_by_color = @pb_items.group_by(&:pb_color).
-      sort_by { |color, items| color.name }.to_h
+      sort_by { |color, items| color&.name }.to_h
 
-    colors_without_thumbnails =
+    colors_without_thumbnails = @pb_items_by_color.keys.
-      @pb_items_by_color.keys.reject(&:pb_item_thumbnail_url?)
+      select(&:present?).reject(&:pb_item_thumbnail_url?)
 
     @pb_color_pet_types = colors_without_thumbnails.map do |color|
       # Infer the ideal species from the first item we can, then try to find a

app/views/items/sources.html.haml

@@ -64,27 +64,33 @@
 		%table.item-list{"data-group-type": "bundle"}
 			%thead
 				%td.thumbnail-cell
-					- if color.pb_item_thumbnail_url?
+					- if color&.pb_item_thumbnail_url?
 						= image_tag color.pb_item_thumbnail_url,
 							alt: "Item thumbnail for #{color.pb_item_name}"
-					- else
+					- elsif color
 						= image_tag pet_type_image_url(@pb_color_pet_types[color], size: :face),
 							srcset: ["#{pet_type_image_url(@pb_color_pet_types[color], size: :face_2x)} 2x"],
 							alt: @pb_color_pet_types[color].human_name
 				%th
-					#{color.pb_item_name || color.name.humanize}
+					- if color
+						#{color.pb_item_name || color.name.humanize}
+					- else
+						??? New color
 					(#{pluralize items.size, "item"})
 				%td.actions-cell
-					- if color.pb_item_name?
+					- if color&.pb_item_name?
 						= button_link_to "Shops",
 							shop_wizard_url_for(color.pb_item_name),
 							target: "_blank", icon: search_icon
 						= button_link_to "Trades",
 							trading_post_url_for(color.pb_item_name),
 							target: "_blank", icon: search_icon
-					- else
+					- elsif color
 						.special-color-explanation
 							Get via Lab Ray, morphing potions, etc.
+					- else
+						.special-color-explanation
+							We don't have info about this color yet.
 			%tbody
 				- items.each do |item|
 					= render "item_list_row", item: