doc: updates examples to use NULL

Examples in the N-API doc used a mix of nullptr and NULL.
We should be consistent and because N-API is a 'C' API I believe
using NULL is better.  This will avoid any potential confusion
as to whether N-API can be used with plain C.

PR-URL: https://github.com/nodejs/node/pull/18008


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>