Fix `String` ctor with `hibyte` for empty strings.

Mark empty strings as compressed for any value of `hibyte`.

Test: New test in 021-string2.
Test: testrunner.py --host --optimizing -t 021-string2
Bug: 250530521
Change-Id: Ie129fba599a24021eda0a4a81a39fd3fc3769597
2 files changed