url: add return value to ToUnicode/ToAscii stubs

This fixes compilation errors like:

  node\src\node_url.cc(134) : error C4716: 'node::url::ToUnicode': must
  return a value

PR-URL: https://github.com/nodejs/node/pull/10893


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michal Zasso <targos@protonmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>