On null changes serialize the id as null

Currently it's serialized as "None" (the string).

Change-Id: I75a8aeb6b11d5700f03baef95527016877ff8f92
diff --git a/tests/test_scheduler.py b/tests/test_scheduler.py
index 48baaef..a92efea 100644
--- a/tests/test_scheduler.py
+++ b/tests/test_scheduler.py
@@ -2558,7 +2558,7 @@
             for q in p['change_queues']:
                 for head in q['heads']:
                     for change in head:
-                        self.assertEqual(change['id'], 'None')
+                        self.assertEqual(change['id'], None)
                         for job in change['jobs']:
                             status_jobs.add(job['name'])
         self.assertIn('project-bitrot-stable-old', status_jobs)
diff --git a/zuul/model.py b/zuul/model.py
index 7fb81bc..440f3ba 100644
--- a/zuul/model.py
+++ b/zuul/model.py
@@ -722,11 +722,8 @@
 class NullChange(Changeish):
     is_reportable = False
 
-    def __init__(self, project):
-        super(NullChange, self).__init__(project)
-
     def _id(self):
-        return 'None'
+        return None
 
     def equals(self, other):
         return False