The XSLT portal includes a shortDescription element in the site information in the raw XML it outputs. The element is always empty, though, even when there is a short description set for the site.
The problem is that XsltRenderContext is trying to pull the shortDescription from the site map object, but the shortDescription isn't added to the site map in PortalSiteHelperImpl on the default portal side, so it is always empty.
I'm attaching a patch that simply adds it to the map created in PortalSiteHelperImpl. Tested in 2.8.x.