test: cover property n-api null cases

Add test coverage for passing `NULL` to each parameter of
`napi.*(propert|element)` and `napi_set_prototype`. In the case of
`napi_define_properties` also test setting various initializer fields
to `NULL`.

PR-URL: #31488
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

Author: Gabriel Schulhof

test/js-native-api/test_object/binding.gyp

@@ -5,6 +5,7 @@
       "sources": [
         "../common.c",
         "../entry_point.c",
+        "test_null.c",
         "test_object.c"
       ]
     }