delayed jobs: fix exception on index page.

test/factories/delayed_job.rb

@@ -0,0 +1,5 @@
+FactoryBot.define do
+  factory :delayed_job, class: Delayed::Job do
+    handler { "" }
+  end
+end

test/functional/delayed_jobs_controller_test.rb

@@ -4,6 +4,7 @@ class DelayedJobsControllerTest < ActionDispatch::IntegrationTest
   context "The delayed jobs controller" do
     context "index action" do
       should "render" do
+        create(:delayed_job)
         get delayed_jobs_path
         assert_response :success
       end