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

appengine/findit/waterfall/monitor_try_job_pipeline.py

Index: appengine/findit/waterfall/monitor_try_job_pipeline.py
diff --git a/appengine/findit/waterfall/monitor_try_job_pipeline.py b/appengine/findit/waterfall/monitor_try_job_pipeline.py
index 89785afdc5bdd99f31b64b0461f669bb0770a4d3..cf8b83a681cf2de167f0c5a5ba6a5c10b981275d 100644
--- a/appengine/findit/waterfall/monitor_try_job_pipeline.py
+++ b/appengine/findit/waterfall/monitor_try_job_pipeline.py
@@ -218,7 +218,8 @@ class MonitorTryJobPipeline(BasePipeline):
   # 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')
@@ -289,4 +290,4 @@ class MonitorTryJobPipeline(BasePipeline):
       # 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