python-adb: Fix bug with shell command out.

bleh

TBR=maruel@chromium.org
BUG=

Review URL: https://codereview.chromium.org/2539543009 .

adb/contrib/high.py

diff --git a/adb/contrib/high.py b/adb/contrib/high.py
index 645d7c2..f1946d7 100644
--- a/adb/contrib/high.py
+++ b/adb/contrib/high.py
@@ -144,7 +144,7 @@
     # 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