Fix crash on drag and drop from xwayland to wayland clients

During DataDevice migration we moved dataDevicesForSurface away from the
existing template macro due to inheritance reasons at the time.

The SeatInterface::setDragTarget() is used with null surfaces

src/wayland/seat_interface.cpp

@@ -234,6 +234,9 @@ QVector<TouchInterface *> SeatInterface::Private::touchsForSurface(SurfaceInterf
 
 QVector<DataDeviceInterface *> SeatInterface::Private::dataDevicesForSurface(SurfaceInterface *surface) const
 {
+    if (!surface) {
+        return {};
+    }
     QVector<DataDeviceInterface *> primarySelectionDevices;
     for (auto it = dataDevices.constBegin(); it != dataDevices.constEnd(); ++it) {
         if ((*it)->client() == *surface->client()) {