QStringBuilder does not necessarily preserve nullness

Another variation of QTBUG-114238:

QString s; 
QVERIFY(s.isNull());

QString r1 = s % s;
QVERIFY(r1.isNull());
QString r2 = s % s % s;
QVERIFY(r2.isNull()); // FAIL

r1 is null because of the special case of QStringBuilder between QStrings. But r2 isn't null, because QStringBuilder builds a 0-length uninitialized string (using QString result(0, Qt::Uninitialized), and that constructor never creates null strings.