node-api: avoid crashing on passed-in null string

When `napi_create_string_*` receives a null pointer as its second
argument, it must null-check it before passing it into V8, otherwise a
crash will occur.

Signed-off-by: Gabriel Schulhof <gabrielschulhof@gmail.com>
PR-URL: https://github.com/nodejs/node/pull/38923


Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>