(cherry picked from commit 7a17c654d293c4afaf3086dc94e8cd4bceac48b1;
amended to resolve minor conflict in TestUtf16.cpp due to us not
(yet?) having `TEST_CASE(null_view)`. Also amended to make the new
method not call simdutf -- it's now also inefficient, but at least
the inefficient code is now only in one place instead of in several)