Prevent creating the Mapbox GL node map if size is empty

Change-Id: I3fe2c7fecd8cf8036f84fed5c4875e39d937c9cd
Reviewed-by: Paolo Angelelli <paolo.angelelli@qt.io>

src/plugins/geoservices/mapboxgl/qgeomapmapboxgl.cpp

@@ -91,6 +91,11 @@ QSGNode *QGeoMapMapboxGLPrivate::updateSceneGraph(QSGNode *node, QQuickWindow *w
 {
     Q_Q(QGeoMapMapboxGL);
 
+    if (m_viewportSize.isEmpty()) {
+        delete node;
+        return 0;
+    }
+
     QMapboxGL *map = 0;
     if (m_useFBO) {
         if (!node) {