Test that url is nil when not provided for author

spec/jekyll_seo_tag/json_ld_drop_spec.rb

@@ -69,6 +69,7 @@ RSpec.describe Jekyll::SeoTag::JSONLDDrop do
           expect(subject["author"]).to have_key("name")
           expect(subject["author"]["name"]).to be_a(String)
           expect(subject["author"]["name"]).to eql("author")
+          expect(subject["author"]["url"]).to be nil
         end
       end
     end