On null changes serialize the id as null

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

Change-Id: I75a8aeb6b11d5700f03baef95527016877ff8f92

tests/test_scheduler.py

@@ -2558,7 +2558,7 @@ class TestScheduler(testtools.TestCase):
             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)

zuul/model.py

@@ -722,11 +722,8 @@ class Ref(Changeish):
 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