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

appengine/findit/waterfall/monitor_try_job_pipeline.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 time
 
 from google.appengine.ext import ndb
 
@@ skipping 200 matching lines @@
 
     if status == BuildbucketBuild.STARTED:
       try_job_result.status = analysis_status.RUNNING
     try_job_result.put()
     return result_to_update
 
   # Arguments number differs from overridden method - pylint: disable=W0221
   # TODO(chanli): Handle try job for test failures later.
   def run(
       self, master_name, builder_name, build_number, try_job_type, try_job_id):
-    assert try_job_id
+    if not try_job_id:
+      return None
 
     timeout_hours = waterfall_config.GetTryJobSettings().get(
         'job_timeout_hours')
     default_pipeline_wait_seconds = waterfall_config.GetTryJobSettings().get(
         'server_query_interval_seconds')
     max_error_times = waterfall_config.GetTryJobSettings().get(
         'allowed_response_error_times')
     pipeline_wait_seconds = default_pipeline_wait_seconds
     allowed_response_error_times = max_error_times
 
@@ skipping 50 matching lines @@
         _UpdateTryJobMetadata(try_job_data, start_time, build, error, True)
         # Explicitly abort the whole pipeline.
         raise pipeline.Abort(
             'Try job %s timed out after %d hours.' % (
                 try_job_id, timeout_hours))
 
       # Ensure last_buildbucket_response is always the most recent
       # whenever available during intermediate queries.
       _UpdateLastBuildbucketResponse(try_job_data, build)
 
-      time.sleep(pipeline_wait_seconds)  # pragma: no cover
+      time.sleep(pipeline_wait_seconds)  # pragma: no cover