fix: try to copy children where there is only one "output" object

packages/myst-compat/src/upgrade/version_1_2.ts

@@ -11,12 +11,14 @@ export function upgrade(ast: Parent) {
     // 2. lift `identifier` and `html_id` labels to `Outputs`
     // 3. lift `visibility` to `Outputs`
     // assert node.children.length === 1
+    const children = node.data?.length === 1 ? [...(node.children as any[])] : [];
     const outputsChildren = (node.data ?? []).map((outputData) => {
       const result: Output2 = {
         type: 'output',
         jupyter_data: outputData,
-        children: [], // FIXME: ignoring children here
+        children, // FIXME: ignoring children here
       };
+      children.length = 0;
       return result;
     });