test: use NULL instead of 0 in common.h

This commit updates the macros in test/addons-napi/common.h to use NULL
instead of 0. This is very minor, but I had to look twice while going
through the code and finding what the macro was doing and comparing
with the struct definition. Using NULL makes it a little clearer I
think.

PR-URL: https://github.com/nodejs/node/pull/24104


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>