[Findit] Make sure not use empty list or dict as default value.

appengine/findit/model/wf_try_job.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 google.appengine.ext import ndb
 
 from model.base_build_model import BaseBuildModel
 from model import analysis_status
 
 
 class WfTryJob(BaseBuildModel):
   """Represents a try job results for a failed build.
 
   'Wf' is short for waterfall.
   """
   # A list of dict containing results and urls of each try job for compile.
-  compile_results = ndb.JsonProperty(default=[], indexed=False, compressed=True)
+  compile_results = ndb.JsonProperty(indexed=False, compressed=True)
 
   # A list of dict containing results and urls of each try job for test.
-  test_results = ndb.JsonProperty(default=[], indexed=False, compressed=True)
+  test_results = ndb.JsonProperty(indexed=False, compressed=True)
 
   # The status of the try job.
   status = ndb.IntegerProperty(
       default=analysis_status.PENDING, indexed=False)
 
   # A list of try job IDs associated with each try job for collecting metadata.
-  try_job_ids = ndb.JsonProperty(default=[], indexed=False, compressed=True)
+  try_job_ids = ndb.JsonProperty(indexed=False, compressed=True)
 
   @staticmethod
   def _CreateKey(master_name, builder_name, build_number):  # pragma: no cover
     return ndb.Key('WfTryJob',
                    BaseBuildModel.CreateBuildId(
                        master_name, builder_name, build_number))
 
   @staticmethod
   def Create(master_name, builder_name, build_number):  # pragma: no cover
-    return WfTryJob(
+    try_job = WfTryJob(
         key=WfTryJob._CreateKey(master_name, builder_name, build_number))
+    try_job.compile_results = try_job.compile_results or []
+    try_job.test_results = try_job.test_results or []
+    try_job.try_job_ids = try_job.try_job_ids or []
+    return try_job
 
   @staticmethod
   def Get(master_name, builder_name, build_number):  # pragma: no cover
     return WfTryJob._CreateKey(
         master_name, builder_name, build_number).get()
 
   @property
   def completed(self):
     return self.status in (
         analysis_status.COMPLETED, analysis_status.ERROR)
 
   @property
   def failed(self):
     return self.status == analysis_status.ERROR