Fix logspam when battery sysfs file not available.
Fix logspam when battery sysfs file not available.

This was a regression caused by a recent change to use String8
instead of char*.  We missed the implicit null check here.

Bug: 9377604
Change-Id: I7eff138096622c47b5d45678010373dc82138384

xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base