Specialize `DefaultEmptyFn` for `std::string`. am: 78ed108510 am: 56de9eb0a1 am: 338b432397 am: d3f6a13d4f am: a47d4a5170

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

Change-Id: I1f63399e492e45764b99b3506afa84600c653c84
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>