do not error out if TOC parent does not have attribs (#17246)

lib/get-mini-toc-items.js

@@ -16,7 +16,7 @@ module.exports = function getMiniTocItems (html, maxHeadingLevel = 3) {
   const items = headings
     .get()
     .filter(item => {
-      if (!item.parent) return true
+      if (!item.parent || !item.parent.attribs) return true
       // Hide any items that belong to a hidden div
       const { attribs } = item.parent
       return !('hidden' in attribs)