Change googlebot targetGenerator to work with empty _escaped_fragment_.

According to section `3. Handle pages without hash fragments` section of https://developers.google.com/webmasters/ajax-crawling/docs/getting-started, if a ajax page has the meta tag `<meta name="fragment" content="!">`, the crawler will crawl the page with an empty `_escaped_fragment_` parameter, e.g. 'www.example.com?_escaped_fragment_='.