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: nodejs#24104
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/addons-napi/common.h

@@ -54,7 +54,7 @@
   NAPI_CALL_BASE(env, the_call, NAPI_RETVAL_NOTHING)
 
 #define DECLARE_NAPI_PROPERTY(name, func)                                \
-  { (name), 0, (func), 0, 0, 0, napi_default, 0 }
+  { (name), NULL, (func), NULL, NULL, NULL, napi_default, NULL }
 
 #define DECLARE_NAPI_GETTER(name, func)                                  \
-  { (name), 0, 0, (func), 0, 0, napi_default, 0 }
+  { (name), NULL, NULL, (func), NULL, NULL, napi_default, NULL }