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.
Checklist
-
make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
commit message follows commit guidelines
Affected core subsystem(s)
n-api
cc @nodejs/api