Post#has_artist_tag: fix "artist tag is required" triggering incorrectly.

Fixes the "Artist tag is required" upload warning to only trigger when
the source is from a first-party site.

app/models/post.rb

@@ -1793,8 +1793,8 @@ class Post < ApplicationRecord
       return if source !~ %r!\Ahttps?://!
       return if has_tag?("artist_request") || has_tag?("official_art")
       return if tags.any? { |t| t.category == Tag.categories.artist }
+      return if Sources::Strategies.find(source).is_a?(Sources::Strategies::Null)
 
-      site = Sources::Strategies.find(source)
       self.warnings[:base] << "Artist tag is required. Create a new tag with [[artist:<artist_name>]]. Ask on the forum if you need naming help"
     end