Clear invalid QApplication styleOverride

When QStyleFactory::create() is unable to find/create an override style
set with QT_STYLE_OVERRIDE or "-style", clear the styleOverride. Reason
for this is that Qt Quick Controls otherwise tries to use this invalid
style.

Task-number: QTBUG-100563
Pick-to: 6.5
Change-Id: I48fa6c211ce27d902e2eaf90c34cb5694ad7ecfd
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/widgets/kernel/qapplication.cpp

@@ -527,6 +527,8 @@ void QApplicationPrivate::initialize()
                 qWarning("QApplication: invalid style override '%s' passed, ignoring it.\n"
                     "\tAvailable styles: %s", qPrintable(styleOverride),
                     qPrintable(QStyleFactory::keys().join(", "_L1)));
+                // Clear styleOverride so it is not picked by Qt Quick Controls (QTBUG-100563)
+                styleOverride.clear();
             }
         }