Fix #3859: Related tag and find artist don't run when fetch data fails.

Fixes an exception in the artist finder caused by searching for a nil profile_url.

app/logical/sources/strategies/null.rb

@@ -13,8 +13,8 @@ module Sources
         url
       end
 
-      def artist_name
-        nil
+      def canonical_url
+        image_url
       end
 
       def normalized_for_artist_finder?
@@ -38,10 +38,6 @@ module Sources
       def unique_id
         url
       end
-
-      def rewrite(url, headers, data)
-        return [url, headers, data]
-      end
     end
   end
 end