Fix crash in the flakiness dashboard server.

Tools/TestResultServer/model/datastorefile.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 168 matching lines @@
             logging.warning("No data to load.")
             return None
 
         data_futures = [(k, DataEntry.get_async(k)) for k in self._convert_blob_keys(self.data_keys)]
 
         data = []
         for key, future in data_futures:
             result = future.get_result()
             if not result:
                 logging.error("No data found for key: %s.", key)
-                return None
+                # FIXME: This really shouldn't happen, but it seems to be happening in practice.
+                # Figure out how and then change this back to returning None.
+                # In the meantime, return empty string so we at least start collecting
+                # results from new runs even though it'll mean that the old data is lost.
+                # crbug.com/377594
+                return ''
             data.append(result)
 
         self.data = "".join([d.data for d in data])
 
         return self.data