test: cleanup/update test-dgram-empty-packet.js

 * Changed some `var` to `const` and 'let'
 * Changed `==` to `===` for clarity.

PR-URL: https://github.com/nodejs/node/pull/8896


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>