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.

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