fix writing spheres with no mesh data

src/scene/scene.cpp

@@ -443,6 +443,7 @@ static void load_node(Scene &scobj, std::vector<std::string> &errors,
 
             Skeleton &skeleton = new_obj.armature;
             aiNode *arm_node = mesh->mBones[0]->mArmature;
+            if(arm_node) {
                 {
                     aiVector3D t, r, s;
                     arm_node->mTransformation.Decompose(s, r, t);
@@ -486,6 +487,7 @@ static void load_node(Scene &scobj, std::vector<std::string> &errors,
                         build_tree(root, root_node->mChildren[k]);
                 }
             }
+        }
 
         node_to_obj[node] = new_obj.id();
         scobj.add(std::move(new_obj));
@@ -919,7 +921,7 @@ std::string Scene::write(std::string file, const Camera &render_cam,
                     face_idx++;
                 }
 
-            } else if(obj.opt.shape_type == PT::Shape_Type::none) {
+            } else {
 
                 const auto &verts = obj.mesh().verts();
                 const auto &elems = obj.mesh().indices();
@@ -946,6 +948,7 @@ std::string Scene::write(std::string file, const Camera &render_cam,
                     face.mIndices[1] = elems[i + 1];
                     face.mIndices[2] = elems[i + 2];
                 }
+
             }
 
             std::string name(obj.opt.name);