[5.7] Fix nested whereDoesntHave()

[staudenmeir]

doesntHave() and whereDoesntHave() don't work properly with nested relationships:

User::whereDoesntHave('posts.comments', function ($query) {
    $query->where('spam', true);
})->get());

Eloquent applies the not exists clause to the innermost relationship instead of the outermost:

select * from "users" where exists (
  select * from "posts" where "users"."id" = "posts"."user_id" and not exists (
    select * from "comments" where "posts"."id" = "comments"."post_id" and "spam" = 1
  )
)

This causes incorrect results (fiddle):

• It returns users that have posts with spam comments and posts without spam comments (user 1).
• It doesn't return users that don't have any posts at all (user 2).

You get the correct query when you nest the closures yourself:

User::whereDoesntHave('posts', function ($query) {
    $query->whereHas('comments', function ($query) {
        $query->where('spam', true);
    });
})->get();

select * from "users" where not exists (
  select * from "posts" where "users"."id" = "posts"."user_id" and exists (
    select * from "comments" where "posts"."id" = "comments"."post_id" and "spam" = 1
  )
)

Fixes #26222.