Specialize `DefaultEmptyFn` for `std::string`. am: 78ed108510

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

Change-Id: Iad4b10e9fa2c181df5e1bcda83f521844dfc3c1e
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>