Make timestamps (created_at, updated_at) NOT NULL

These values are always set by ActiveRecord and so will never be stored
as NULL (or Ruby nil).

Overall, improves data integrity by hardening the schema constraints.

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 null: true
+      table.timestamps null: false
     end
 
     add_index :delayed_jobs, [:priority, :run_at], name: "delayed_jobs_priority"

lib/generators/delayed_job/templates/timestamps_not_null.rb

@@ -0,0 +1,6 @@
+class TimestampsNotNull < ActiveRecord::Migration<%= migration_version %>
+  def change
+    change_column_null :delayed_jobs, :created_at, false
+    change_column_null :delayed_jobs, :updated_at, false
+  end
+end

lib/generators/delayed_job/upgrade_generator.rb

@@ -14,6 +14,11 @@ def create_migration_file
         "db/migrate/add_queue_to_delayed_jobs.rb",
         migration_version: migration_version
       )
+      migration_template(
+        "timestamps_not_null.rb",
+        "db/migrate/timestamps_not_null.rb",
+        migration_version: migration_version
+      )
     end
   end
 end