Fusion style - avoid a warning

... printed by QPainter when it gets a pixmap with one or both of
dimensions equal to 0 (and a nullptr as a paintEngine).

Task-number: QTBUG-71806
Change-Id: I978f56c843daab307042e34390fc33f338ce8cf5
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/widgets/styles/qfusionstyle.cpp

@@ -247,6 +247,9 @@ static QLinearGradient qt_fusion_gradient(const QRect &rect, const QBrush &baseC
 
 static void qt_fusion_draw_arrow(Qt::ArrowType type, QPainter *painter, const QStyleOption *option, const QRect &rect, const QColor &color)
 {
+    if (rect.isEmpty())
+        return;
+
     const int arrowWidth = QStyleHelper::dpiScaled(14);
     const int arrowHeight = QStyleHelper::dpiScaled(8);