A cursor to fetch the next page, or null when the page is the last.

The guarantee: nextCursor is non-null if and only if the page is non-terminal — including a page that returned zero rows while further matching rows remain. It is never derived from rows.length.

A backend produces the cursor by one of two mechanisms. A row-based backend fetches limit + 1 rows and sets nextCursor to null precisely when it fetched no more than limit — a cursor that returned an empty page would otherwise be indistinguishable from a genuine last page. A token-based backend carries the server's own continuation key inside the opaque token and sets nextCursor to null precisely when the server answered without one; on such a backend a filtered page can return fewer rows than limit — or none — and still not be the last page.