`mdc-element` without children isn't handled correctly

[Ni55aN]

Reproduction

1. Open https://codesandbox.io/p/devbox/sad-euler-qft5yl

2. Make sure content/*.md has list with empty item

# Test

- a
- b
- c
- 

3. Go to /llms-ffull.txt

Logs

 ERROR  [request error] [unhandled] [GET] http://localhost:3000/llms-full.txt

 
ℹ Error: Cannot read properties of undefined (reading '0')

 ⁃ at extractLeadingCheckbox (node_modules/hast-util-to-mdast/lib/handlers/li.js:117:29)

   112 ┃   *
   113 ┃   * @param {Readonly<Element>} node
   114 ┃   * @returns {ExtractResult}
   115 ┃   */
   116 ┃  function extractLeadingCheckbox(node) {
 ❯ 117 ┃    const head = node.children[0]
   118 ┃  
   119 ┃    if (
   120 ┃      head &&
   121 ┃      head.type === 'element' &&
   122 ┃      head.tagName === 'input' &&

 ⁃ at Object.li (node_modules/hast-util-to-mdast/lib/handlers/li.js:30:28)
 ⁃ at mdc-element (node_modules/@nuxtjs/mdc/dist/runtime/stringify/mdc-remark.js:98:42)
 ⁃ at Object.one (node_modules/hast-util-to-mdast/lib/state.js:255:40)
 ⁃ at Object.all (node_modules/hast-util-to-mdast/lib/state.js:220:14)
 ⁃ at Object.list (node_modules/hast-util-to-mdast/lib/handlers/list.js:19:50)
 ⁃ at mdc-element (node_modules/@nuxtjs/mdc/dist/runtime/stringify/mdc-remark.js:98:42)
 ⁃ at Object.one (node_modules/hast-util-to-mdast/lib/state.js:255:40)
 ⁃ at Object.all (node_modules/hast-util-to-mdast/lib/state.js:220:14)
 ⁃ at Object.root (node_modules/hast-util-to-mdast/lib/handlers/root.js:18:24)

[CAUSE]
TypeError {
  stack: "Cannot read properties of undefined (reading '0')\n" +
  'at extractLeadingCheckbox (./node_modules/hast-util-to-mdast/lib/handlers/li.js:117:29)\n' +
  'at Object.li (./node_modules/hast-util-to-mdast/lib/handlers/li.js:30:28)\n' +
  'at mdc-element (./node_modules/@nuxtjs/mdc/dist/runtime/stringify/mdc-remark.js:98:42)\n' +
  'at Object.one (./node_modules/hast-util-to-mdast/lib/state.js:255:40)\n' +
  'at Object.all (./node_modules/hast-util-to-mdast/lib/state.js:220:14)\n' +
  'at Object.list (./node_modules/hast-util-to-mdast/lib/handlers/list.js:19:50)\n' +
  'at mdc-element (./node_modules/@nuxtjs/mdc/dist/runtime/stringify/mdc-remark.js:98:42)\n' +
  'at Object.one (./node_modules/hast-util-to-mdast/lib/state.js:255:40)\n' +
  'at Object.all (/home/n'... 201 more characters,
  message: "Cannot read properties of undefined (reading '0')",
}