I think you can do some cool things when you set classes. A colon :
indicates 'else'.
{%
set classes = [
item.is_expanded ? 'is-expanded' : 'not-expanded',
item.is_collapsed ? 'is-collapsed' : 'not-collapsed',
item.in_active_trail ? 'active',
item.below ? 'has-children': 'no-childrenchildren',
]
%}
EDIT: Added ', after no-children