url: fix treatment of some values as non-empty

In addition to null, undefined and the empty string
are treated as empty (removing the component from the url).

The string '#' is treated same as empty values when
setting .hash.

The string '?' is treated same as empty values when
setting .search.

PR-URL: https://github.com/iojs/io.js/pull/1589
Fixes: https://github.com/iojs/io.js/issues/1588


Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>