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

appengine/findit/waterfall/test/monitor_try_job_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 datetime import datetime
 import json
 import mock
 import time
 
 from common.waterfall import buildbucket_client
@@ skipping 460 matching lines @@
     }
 
     expected_error_dict = {
         'message': 'No result report was found.',
         'reason': MonitorTryJobPipeline.UNKNOWN
     }
 
     self.assertEqual(
         monitor_try_job_pipeline._GetError(build_response, None, False),
         (expected_error_dict, try_job_error.UNKNOWN))
+
+  def testReturnNoneIfNoTryJobId(self):
+    master_name = 'm'
+    builder_name = 'b'
+    build_number = 1
+    pipeline = MonitorTryJobPipeline()
+    test_result = pipeline.run(
+        master_name, builder_name, build_number, failure_type.TEST,
+        None)
+    self.assertIsNone(test_result)