Check for empty output in adb_wait_for_charge.

platform_tools/android/bin/adb_wait_for_charge

 #!/bin/bash
 #
 # Wait for the device to be charged enough for testing.
 
 SCRIPT_DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
 SKIP_TOOLCHAIN_SETUP="true"
 source $SCRIPT_DIR/android_setup.sh
 source $SCRIPT_DIR/utils/setup_adb.sh
 
 # Helper function used by get_battery_level. Parses the battery level from
@@ skipping 36 matching lines @@
   if [ -n "$RV" ]; then
     echo "$RV"
     return
   fi
 
   echo "Battery level fallback..." 1>&2
 
   STATS="$($ADB $DEVICE_SERIAL shell dumpsys battery)"
   SPLIT=( $STATS )
   RV="$(_parse_battery_level ${SPLIT[@]})"
-  if [ "$RV" != "-1" ]; then
+  if [ -n "$RV" ] && [ "$RV" != "-1" ]; then
     echo "$RV"
     return
   fi
 
   echo "Could not determine battery level!" 1>&2
   # Just exit to prevent hanging forever or failing the build.
   echo "0"
 }
 
 # Wait for battery charge.
 DESIRED_BATTERY_LEVEL=80
 CURRENT_BATTERY_LEVEL="$(get_battery_level)"
 while [ "${CURRENT_BATTERY_LEVEL}" -lt "${DESIRED_BATTERY_LEVEL}" ]; do
   echo "Battery level is ${CURRENT_BATTERY_LEVEL}; waiting to charge to ${DESIRED_BATTERY_LEVEL}"
   sleep 5
   CURRENT_BATTERY_LEVEL="$(get_battery_level)"
 done
 
 echo "Charged!"