Systematic testing in tst_QStringApiSymmetry revealed a bug in
QStringRef::toLatin1(): a null input did not result in a null output,
but an empty one. This is fixed, for consistency with
QString::toLatin1(), and QString(Ref)::toUtf8(), which behaved
correctly already.
The same bug was found in QString(Ref)::toLocal8Bit(), which is
particularly hideous, as it's documented to fall back to toLatin1(),
which preserves null inputs. Fixed, too.
[ChangeLog][QtCore][QString] toLocal8Bit() now preserves nullness of
the input QString (outputs null QByteArray).
[ChangeLog][QtCore][QStringRef] toLocal8Bit() and toLatin1() now
preserve nullness of the input QStringRef (output null QByteArrays).
Change-Id: I7026211922c287e03d07e89edbad2987aa646e51
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
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