Fix deprecation in ActiveRecord::Migration#timestamp

lib/generators/delayed_job/templates/migration.rb

@@ -10,7 +10,7 @@ def self.up
       table.datetime :failed_at                        # Set when all retries have failed (actually, by default, the record is deleted instead)
       table.string :locked_by                          # Who is working on this object (if locked)
       table.string :queue                              # The name of the queue this job is in
-      table.timestamps
+      table.timestamps null: true
     end
 
     add_index :delayed_jobs, [:priority, :run_at], name: "delayed_jobs_priority"