test: refactor test-http-write-empty-string to use arrow functions

In `test/parallel/test-http-write-empty-string.js`, callbacks use
anonymous closure functions. Replace them with arrow functions.

PR-URL: https://github.com/nodejs/node/pull/24483


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>