src: fix debug crash handling null strings

When internal debug is enabled, output null strings as
"(null)" rather than crashing, matching glibc's behavior.

PR-URL: https://github.com/nodejs/node/pull/31523


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>