Gui: modernize C++: return braced init list

src/Gui/BitmapFactory.cpp

@@ -239,7 +239,7 @@ bool BitmapFactoryInst::loadPixmap(const QString& filename, QPixmap& icon) const
 QPixmap BitmapFactoryInst::pixmap(const char* name) const
 {
     if (!name || *name == '\0')
-        return QPixmap();
+        return {};
 
     // as very first test check whether the pixmap is in the cache
     QMap<std::string, QPixmap>::Iterator it = d->xpmCache.find(name);