Custom title page does not display in PDF bookmarks if heading styles are not used

[mcgratay]

Description

As described in #596, using heading styles in the custom title page causes information to display and nest illogically in PDF bookmarks.

If heading styles are not used in the editor for the custom title page, however, the page does not appear in the PDF bookmarks at all.

The custom title page should appear in the bookmark list as the label "Title page", the same as recommended for auto-generated title pages in #595

Steps to Reproduce

1. Create a new front matter post
2. Select front matter type "Title page"
3. Add content to the editor without adding any heading styles
4. Save and export to PDF
5. View PDF bookmarks

Expected behavior: Custom title page can always be identified and navigated to from PDF bookmarks

Actual behavior: Custom title page does not appear in bookmark list when heading styles are not used in the post