Sitemap tree is not displayed when root sitemap points to an xpage document

[Regression]

In 15.4.0, sitemap items disappear when there are no xpages present, and the root sitemap item (home) points to an xpage document using hst:relativecontentpath property.

1. Create a few pages under the root sitemap (do not create any xpages).
2. Create an xpage document 'XpageDoc'.
3. Point the root sitemap item to the new xpage document using hst:relativecontentpath property.
4. Navigate to the channel to find the sitemap missing under the root sitemap item.
5. However, once you create at least one experience page, the sitemap is displayed again.

After step 4 above, if you observe the network response in 15.4.0, it has experiencePage : true &  expandable : false

In 14.7.5 however, the response has experiencePage : false and the sitemap is displayed.