Replace QString::null with default constructed QString

QString::null is deprecated since Qt 5.9.

Change-Id: Ib84f338ed8cecaee0c164191fb580c851bd84ab4
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/qmljseditor/quicktoolbar.cpp

@@ -96,7 +96,7 @@ QuickToolBar::QuickToolBar()
                << QLatin1String("font.italic")
                << QLatin1String("font.underline")
                << QLatin1String("font.strikeout")
-               << QString::null
+               << QString()
                << QLatin1String("states")
                << QLatin1String("transitions")
                ;