Bug fix: null string will cause exception in checkStringInAdbCommandOutput

Bug: 192036015
Test: atest com.android.settings.fuelgauge.batterysaver.BatterySaverButtonPreferenceControllerComponentTest#test_check_battery_saver_button
Change-Id: Ifc73c8c9d6b5fabca6af5ea604f23a789b99d217
https://atp.googleplex.com/test_runs/128222905
1 file changed