How to add nested twig attributes?

I want to integrate the guestbook plugin into future2021 theme and render the existing guestbook entries as blog-items. blog_item.html.twig accesses the attribute page.header.continue_link. I’d like to make use of that attribute, but the guestbook entry doesn’t contain it, of course.

I know that I can set simple attributes using {% set child = child|merge({'title':}) %} and pass child as page to blog_item.html.twig. But how can I set nested child.header.continue_link to false when there’s no header attribute in child?