python-adb: Fix bug with shell command out.

adb/contrib/high.py

 # coding=utf-8
 # Copyright 2015 Google Inc. All rights reserved.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
 # You may obtain a copy of the License at
 #
 #     http://www.apache.org/licenses/LICENSE-2.0
 #
 # Unless required by applicable law or agreed to in writing, software
@@ skipping 126 matching lines @@
           continue
         key, value = line.split(u'=', 1)
         properties[key] = value
 
     mode, _, _ = device.Stat('/system/xbin/su')
     has_su = bool(mode)
     # Stat'ing /system/xbin/su directly from adbd outside of a shell can fail
     # due to SELinux permission errors. Try again by executing su via shell.
     if not has_su:
       out, exit_code = device.Shell('/system/xbin/su root whoami')
-      if not exit_code and out.strip() == 'root':
+      if not exit_code and out and out.strip() == 'root':
         has_su = True
 
     available_governors = KNOWN_CPU_SCALING_GOVERNOR_VALUES
     out = device.PullContent(
         '/sys/devices/system/cpu/cpu0/cpufreq/scaling_available_governors')
     if out:
       available_governors = sorted(i for i in out.split())
       assert set(available_governors).intersection(
           KNOWN_CPU_SCALING_GOVERNOR_VALUES), available_governors
 
@@ skipping 844 matching lines @@
     return out
 
   @classmethod
   def _Connect(cls, constructor, **kwargs):
     """Called by either ConnectDevice or Connect."""
     if not kwargs.get('rsa_keys'):
       with _ADB_KEYS_LOCK:
         kwargs['rsa_keys'] = _ADB_KEYS[:]
     device = constructor(**kwargs)
     return HighDevice(device, _InitCache(device))