Avoid crash in scene importer when the mesh is empty.

editor/import/resource_importer_scene.cpp

@@ -342,6 +342,7 @@ static String _fixstr(const String &p_what, const String &p_str) {
 
 static void _pre_gen_shape_list(Ref<ImporterMesh> &mesh, Vector<Ref<Shape3D>> &r_shape_list, bool p_convex) {
 	ERR_FAIL_NULL_MSG(mesh, "Cannot generate shape list with null mesh value");
+	ERR_FAIL_NULL_MSG(mesh->get_mesh(), "Cannot generate shape list with null mesh value");
 	if (!p_convex) {
 		Ref<Shape3D> shape = mesh->create_trimesh_shape();
 		r_shape_list.push_back(shape);