[Android] Fix perf test runner in platform mode return value for when devices are blacklisted.

build/android/pylib/local/device/local_device_perf_test_run.py

 # Copyright 2016 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.
 
 import io
 import json
 import logging
 import os
 import pickle
 import shutil
@@ skipping 326 matching lines @@
     # Affinitize the tests.
     self._SplitTestsByAffinity()
     if not self._test_buckets:
       raise local_device_test_run.NoTestsError()
 
     def run_perf_tests(shard_id):
       if device_status.IsBlacklisted(
           str(self._devices[shard_id]), self._env.blacklist):
         logging.warning('Device %s is not active. Will not create shard %s.',
                         str(self._devices[shard_id]), shard_id)
-        return []
+        return None
       s = TestShard(self._env, self._test_instance, self._devices[shard_id],
                     shard_id, self._test_buckets[shard_id],
                     retries=self._env.max_tries, timeout=self._timeout)
       return s.RunTestsOnShard()
 
     device_indices = range(min(len(self._devices), len(self._test_buckets)))
     shards = parallelizer.Parallelizer(device_indices).pMap(run_perf_tests)
     return [x for x in shards.pGet(self._timeout) if x is not None]
 
   # override
@@ skipping 59 matching lines @@
   # override
   def _RunTest(self, _device, _test):
     raise NotImplementedError
 
 
 class TestDictVersionError(Exception):
   pass
 
 class PerfTestRunGetStepsError(Exception):
   pass