test: add two test cases for querystring

+ Check an empty substring: In `querystring`, if the `maxKeys` is 1
  and the state machine finds an empty substring between separators,
  it should return an empty object.
+ Test invalid encoded strings: If provided string is an invalid
  encoded string in `query.parse`, it will not be encoded.

PR-URL: https://github.com/nodejs/node/pull/11481


Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>