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/base.rb

@@ -71,11 +71,11 @@ module Sources
 
       # A link to the artist's profile page on the site.
       def profile_url
-        nil
+        ""
       end
 
       def artist_name
-        raise NotImplementedError
+        ""
       end
 
       def artist_commentary_title
@@ -129,7 +129,8 @@ module Sources
       end
 
       def artists
-        Artist.find_artists(profile_url)
+        url = profile_url.presence || image_url.presence
+        Artist.find_artists(url)
       end
 
       def file_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