Fixes #19996.

This was unearthed by #19400 where `is_subtype(Partial<X>, Partial<X>)`
started returning True - before that `binder.assign_type(expr,
Partial<None>, Partial<None>)` just returned early. If I understand
correctly, that is how Partial should be handled here: we do not want to
push partials to the binder. I do not think we should add a special case
for that (both False and True make some sense for a partial type, I am
not convinced that either one is marginally better), so I just add an
explicit guard to skip adding partial types here.