Non Clickable Nav Link
You just create it as a new attribute with the handle replace_link_with_first_in_nav if it doesn't already exist.
Then you add that to your top level page as an attribute, ticked.
While the item in the nav is still clickable, it'll click through to the first sub-page.
It might be just a personal preference, but I prefer it when navigations don't have some links and some unusable placeholders mixed together.
If that doens't
It has an option to make menu items no-go only hoverable links