. I remember encountering similar issue while playing around with SharePoint 2013 preview. The timer job has a weird bug that sometimes doesn’t generate a sitemap file. Internally timer job replaces “Default Zone” site collection URL with “Internet Zone” site collection URL and if “Internet Zone” hasn’t been set, it tries using value in SPSite.Url which doesn’t has a trailing slash and without that the URL is considered invalid and it is skipped and not added to sitemap file. Because of this bug, there will be nothing in a new sitemap file and hence the file is neither generated nor added to “robots.txt” and the timer job appears to be successful.
Try defining an “Internet zone” and run Sitemap job again to see if it generates sitemap file and append to Robots.txt. Also check ULS logs for any related warnings and/or errors.