test: fix compiler warning in n-api test

Missed in ca786c37. This does
not actually affect the outcome because returning `nullptr` or
`this` from a constructor has the same effect.

PR-URL: https://github.com/nodejs/node/pull/12318


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>