artists: fix exception on show page when artist has invalid URLs.

Fix an exception on the artist show page when the artist entry contained invalid URLs such
as `http://ttp://album.yahoo.co.jp/photos/my/8027988`. Caused by `ArtistUrl#domain`
returning nil for certain invalid URLs, which caused `Artist#sorted_urls` to blow up.

ref: https://danbooru.donmai.us/forum_posts/206488

app/models/artist_url.rb

@@ -72,7 +72,7 @@ class ArtistURL < ApplicationRecord
   end
 
   def domain
-    Danbooru::URL.parse(normalized_url)&.domain
+    Danbooru::URL.parse(normalized_url)&.domain.to_s
   end
 
   def site_name