[Findit] Fix a bug that MonitorTryJobPipeline still runs when no try job scheduled.

appengine/findit/waterfall/test/identify_try_job_culprit_pipeline_test.py

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 from testing_utils import testing
 
 from common.git_repository import GitRepository
 from common.waterfall import failure_type
 from model import analysis_status
 from model import result_status
@@ skipping 938 matching lines @@
     culprits = {
         'r1': {
             'repo_name': 'chromium',
             'revision': 'r1',
         }
     }
 
     identify_try_job_culprit_pipeline._NotifyCulprits('m', 'b', 1, culprits)
     self.assertEqual(1, len(instances))
     self.assertTrue(instances[0].started)
+
+  def testReturnNoneIfNoTryJob(self):
+    master_name = 'm'
+    builder_name = 'b'
+    build_number = 8
+
+    try_job = WfTryJob.Create(master_name, builder_name, build_number).put()
+    pipeline = IdentifyTryJobCulpritPipeline()
+    culprit = pipeline.run(master_name, builder_name, build_number, ['rev1'],
+                 failure_type.TEST, None, None)
+    self.assertIsNone(culprit)
+
+    try_job = WfTryJob.Get(master_name, builder_name, build_number)
+    self.assertEqual(try_job.test_results, [])
+    self.assertEqual(try_job.status, analysis_status.COMPLETED)