Optional struct/list/map fields resolve to `undefined` instead of `null`

[cas--]

While parsing an optional struct I was surprised to find hyparquet was returning undefined instead of null when the struct was assigned null by duckdb.

Reproduction

Tested hyparquet 1.25.6 and 1.26.2 with the following:

import { parquetWriteBuffer } from 'hyparquet-writer'
import { parquetMetadataAsync, parquetReadObjects } from 'hyparquet'

const schema = [
  { name: 'root', num_children: 2 },
  { name: 'scalar', type: 'INT32', repetition_type: 'OPTIONAL' },
  { name: 'struct', num_children: 1, repetition_type: 'OPTIONAL' },
  { name: 'a', type: 'INT32', repetition_type: 'OPTIONAL' },
]

const arrayBuffer = parquetWriteBuffer({
  schema,
  columnData: [
    { name: 'scalar', data: [null] },
    { name: 'struct', data: [null] },
  ],
})

const file = {
  byteLength: arrayBuffer.byteLength,
  slice: (start, end) => arrayBuffer.slice(start, end ?? arrayBuffer.byteLength),
}
const metadata = await parquetMetadataAsync(file)
const rows = await parquetReadObjects({ file, metadata })

rows[0].scalar // null
rows[0].struct // undefined — inconsistent, and drops silently from JSON.stringify

Root cause

The issue is flattenAtDepth in src/assemble.js which unwraps via arr[i][0] and is undefined for an empty array, unlike assembleLists, which explicitly pushes null for absent scalars.

I can create a PR with the fix and updated tests:

--- src/assemble.js
+++ src/assemble.js
@@ -196,7 +196,7 @@ function flattenAtDepth(arr, depth) {
     if (depth) {
       flattenAtDepth(arr[i], depth - 1)
     } else {
-      arr[i] = arr[i][0]
+      arr[i] = arr[i].length ? arr[i][0] : null
     }
   }
 }