test: adding mustCall in test-fs-readfile-empty.js

PR-URL: https://github.com/nodejs/node/pull/27455


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ouyang Yadong <oyydoibh@gmail.com>