test: cases to querystring related to empty string

+ Add cases to `qs.stringify` that return empty string
+ Add cases to `qs.parse` when `sep` or `eq` is empty

PR-URL: https://github.com/nodejs/node/pull/11329


Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>