test: fix args in parallel/test-fs-null-bytes.js

The functions `fs.appendFile()` and `fs.writeFile()`
were being called without the required `data` argument.

Refs: https://github.com/nodejs/node/issues/11595
PR-URL: https://github.com/nodejs/node/pull/11601


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Nikolai Vavilov <vvnicholas@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>