http: always call response.write() callback

Ensure that the callback of `OutgoingMessage.prototype.write()` is
called even when writing empty chunks.

Fixes: https://github.com/nodejs/node/issues/22066

PR-URL: https://github.com/nodejs/node/pull/27709


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>