Specialize `DefaultEmptyFn` for `std::string`. am: 78ed108510 am: 56de9eb0a1 am: 338b432397

Original change: https://android-review.googlesource.com/c/platform/art/+/2225903

Change-Id: I8e34f683f30f4dc7c76b39d4bd4875b7f0626174
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>