In `test/parallel/test-http-write-empty-string.js`, callbacks use
anonymous closure functions. It is safe to replace them with arrow
functions since these callbacks don't contain references to `this`,
`super` or `arguments`. This results in shorter functions.

In `test/parallel/test-http-write-empty-string.js`, the callback passed
to `http.createServer` contains a reference to `this`. Since the object
referenced by `this` doesn't pre-exist, revert the arrow function to an
anonymous closure function as a callback.

Similarly, the callback passed to `server.listen` too contains a
reference to `this`. However, in this case, `this` resolves to a
pre-existing object `server`. Therefore, it is safe to use an arrow
function as a callback as long as `this` is replaced with `server`.

Fix ESLint complaint: "28:42 Unexpected space before function
parentheses space-before-function-paren".