mediawiki.util: Use wgScript in getUrl() when setting query string

mediawiki.util: Use wgScript in getUrl() when setting query string

This makes the behaviour more in line with Title::getLocalURL() in PHP.

* Only use wgArticlePath if there was a non-empty query string.
* Remove needless call to isEmptyObject. This is already done by $.params().
* Add test case "title with empty query string".

Change-Id: I414350a527eea3822ff2843c1c06a925f41a2c03