Fix SitemapNodeMiddleware SITEMAP_ROOT resulting in 404.
Fix SitemapNodeMiddleware SITEMAP_ROOT resulting in 404.

If the node is the SITEMAP_ROOT (a root node in the tree, and has the
special slug that equals SITEMAP_ROOT value) then we need the path to be
an empty string, regardless of the APPEND_SLASH setting.

goodtune/vitriolic,goodtune/vitriolic,goodtune/vitriolic,goodtune/vitriolic