Check for empty output in adb_wait_for_charge.

Example error in https://chromium-swarm.appspot.com/user/task/2f96d6439584e411

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2105813003

Review-Url: https://codereview.chromium.org/2105813003
1 file changed