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

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

Change-Id: I2f22c7843a235263fa212b56895afb8778a94564
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>