n-api: use nullptr instead of NULL in node_api.cc

This commit changes two checks which use NULL to use nullptr.

I'm not very familiar with N-API but wanted to bring this up in case
it was something that was overlooked.

PR-URL: https://github.com/nodejs/node/pull/17276


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Lance Ball <lball@redhat.com>
Reviewed-By: Alexey Orlenko <eaglexrlnk@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: MichaëZasso <targos@protonmail.com>