Fix 500 in test results server when querying testlistjson

Tools/TestResultServer/handlers/testfilehandler.py

 # Copyright (C) 2010 Google Inc. All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 # notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 # copyright notice, this list of conditions and the following disclaimer
@@ skipping 189 matching lines @@
             if not master_data:
                 master_data = master_config.getMasterByMasterName(master)
             if not master_data:
                 self.error(404)
                 return
 
             json, date = self._get_file_content(master_data['url_name'], builder, test_type, build_number, name)
             if json is None:
                 json, date = self._get_file_content(master_data['name'], builder, test_type, build_number, name)
 
-            if test_list_json:
+            if json and test_list_json:
                 json = JsonResults.get_test_list(builder, json)
 
         if json:
             json = _replace_jsonp_callback(json, callback_name)
 
         self._serve_json(json, date)
 
 
 class Upload(webapp2.RequestHandler):
     """Upload test results file to datastore."""
@@ skipping 82 matching lines @@
 
 class UploadForm(webapp2.RequestHandler):
     """Show a form so user can upload a file."""
 
     def get(self):
         template_values = {
             "upload_url": "/testfile/upload",
         }
         self.response.out.write(template.render("templates/uploadform.html",
                                                 template_values))