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

test/unit/sources/null_test.rb

@@ -0,0 +1,32 @@
+require 'test_helper'
+
+module Sources
+  class NullTest < ActiveSupport::TestCase
+    context "A source from an unknown site" do
+      setup do
+        @site = Sources::Strategies.find("http://oremuhax.x0.com/yoro1603.jpg", "http://oremuhax.x0.com/yo125.htm")
+      end
+
+      should "be handled by the null strategy" do
+        assert(@site.is_a?(Sources::Strategies::Null))
+      end
+
+      should "find the metadata" do
+        assert_equal("oremuhax.x0.com", @site.site_name)
+        assert_equal(["http://oremuhax.x0.com/yoro1603.jpg"], @site.image_urls)
+        assert_equal("http://oremuhax.x0.com/yoro1603.jpg", @site.image_url)
+        assert_equal("http://oremuhax.x0.com/yoro1603.jpg", @site.canonical_url)
+        assert_equal("", @site.artist_name)
+        assert_equal("", @site.profile_url)
+        assert_nothing_raised { @site.to_h }
+      end
+
+      should "find the artist" do
+        a1 = FactoryBot.create(:artist, name: "test1", url_string: "http://oremuhax.x0.com")
+        a2 = FactoryBot.create(:artist, name: "test2", url_string: "http://google.com")
+
+        assert_equal([a1], @site.artists)
+      end
+    end
+  end
+end