Fix null ref in github tests

The fake github was producing the wrong number of zeroes.

Change-Id: Ibc2b4d4ca2ed7b8444dfa0ab32032a360592f4a6
diff --git a/tests/unit/test_github_driver.py b/tests/unit/test_github_driver.py
index 4077cca..84fdbde 100644
--- a/tests/unit/test_github_driver.py
+++ b/tests/unit/test_github_driver.py
@@ -110,10 +110,8 @@
 
         build_params = self.builds[0].parameters
         self.assertEqual('refs/tags/newtag', build_params['zuul']['ref'])
-        self.assertEqual('00000000000000000000000000000000',
-                         build_params['zuul']['oldrev'])
+        self.assertFalse('oldrev' in build_params['zuul'])
         self.assertEqual(sha, build_params['zuul']['newrev'])
-
         self.executor_server.hold_jobs_in_build = False
         self.executor_server.release()
         self.waitUntilSettled()