async_hooks: check for empty contexts before removing

This way we don't end up attempting to SetPromiseHooks on contexts that
have already been collected.

Fixes: https://github.com/nodejs/node/issues/39019

PR-URL: https://github.com/nodejs/node/pull/39095


Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>
Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Andrey Pechkurov <apechkurov@gmail.com>
Reviewed-By: Danielle Adams <adamzdanielle@gmail.com>