gralloc960/mali_gralloc_debug.cpp: migrate to String.empty() am: ca8781cc8b

Original change: https://android-review.googlesource.com/c/device/linaro/hikey/+/2765165

Change-Id: I478a40a757f865cd67aa7ba29c81f2da8adb029d
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>