When a needle has length 1 (because it's a QChar/char16_t, or because
it's a string-like of length 1) then an ad-hoc search algorithm is
used. This algorithm had a off-by-one, by not allowing to match at
the last position of a haystack (in case `from` was `haystack.size()`).
That is inconsistent with the general search of substring needles
(and what QByteArray does). Fix that case and amend wrong tests.
This in turn unveiled the fact that the algorithm was unable to cope
with 0-length haystacks (whops), so fix that as well. Drive-by, add a
similar fix for QByteArray.
Amends 6cee204d56.
Pick-to: 6.2
Change-Id: I6b3effc4ecd74bcbcd33dd2e550da2df7bf05ae3
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
f0c2c987e3