Fix `String` ctor with `hibyte` for empty strings. am: 729277fa70 am: 9c74398921

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

Change-Id: Ic511a61d1c46b7139fa98dc755020aa233e93d5f
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>