[Android] Only attempt to rerun tests if some tests run.

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

 # Copyright 2014 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 collections
 import itertools
 import logging
 import os
 import posixpath
 import time
@@ skipping 422 matching lines @@
                 include_stack_symbols=False,
                 wipe_tombstones=True)
             stream_name = 'tombstones_%s_%s' % (
                 time.strftime('%Y%m%dT%H%M%S', time.localtime()),
                 device.serial)
             tombstones_url = tombstones.LogdogTombstones(resolved_tombstones,
                                                          stream_name)
           result.SetTombstonesUrl(tombstones_url)
 
     not_run_tests = set(test).difference(set(r.GetName() for r in results))
-    return results, list(not_run_tests)
+    return results, list(not_run_tests) if results else None
 
   #override
   def TearDown(self):
     @local_device_environment.handle_shard_failures
     def individual_device_tear_down(dev):
       for s in self._servers.get(str(dev), []):
         s.TearDown()
 
       tool = self.GetTool(dev)
       tool.CleanUpEnvironment()
 
     self._env.parallel_devices.pMap(individual_device_tear_down)