[Android] Add a check in sharder for an empty test list.

build/android/pylib/base/shard.py

 # Copyright (c) 2013 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.
 
 """Implements test sharding logic."""
 
 import logging
 import threading
 
 from pylib import android_commands
@@ skipping 264 matching lines @@
     build_type: either 'Debug' or 'Release'.
     test_timeout: watchdog timeout in seconds for running tests, defaults to the
       default timeout.
     setup_timeout: watchdog timeout in seconds for creating and cleaning up
       test runners, defaults to the default timeout.
     num_retries: number of retries for a test.
 
   Returns:
     A base_test_result.TestRunResults object.
   """
+  if not tests:
+    logging.warning('No tests to run.')
+    return base_test_result.TestRunResults()
+
   logging.info('Will run %d tests: %s', len(tests), str(tests))
   forwarder.Forwarder.KillHost(build_type)
   runners = _CreateRunners(runner_factory, devices, setup_timeout)
   try:
     return _RunAllTests(runners, tests, num_retries, test_timeout)
   finally:
     try:
       _TearDownRunners(runners, setup_timeout)
     except android_commands.errors.DeviceUnresponsiveError as e:
       logging.warning('Device unresponsive during TearDown: [%s]', e)
     finally:
       forwarder.Forwarder.KillHost(build_type)