Google Analytics configurations are not saved when using short URLs without the bin part

When the user has configured his wiki to remove the /xwiki/bin/ part of the URLs (see http://platform.xwiki.org/xwiki/bin/view/Main/ShortURLs), the configuration for Google Analytics cannot be saved anymore on the Administration section.
The form to submit the Google Analytics changes on XWiki.GoogleAnalyticsCode should use the empty string instead of $xwiki.requestURL for the action attribute.