Fix `String` ctor with `hibyte` for empty strings. am: 729277fa70

Original change: https://android-review.googlesource.com/c/platform/art/+/2238520

Change-Id: Id7ef70513787ed864d6011d2472816fafff57522
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>