[Backport 3.1.3] HST Link Creation fails on root path for Plain REST mount

If you configure a plain jaxrs rest api mount and if you create a link from normal site jsp template (in root mount, not rest api mount) like the following, it fails to create a proper link (in template of hst:root mount):

<hst:link path='/' mount='api' />

, where 'api' is the mount alias as follows:

`- hst:root { hst:mountapi = "api" }
    `- api { hst:alias="api" }

In the api mount, there's no sitemap configuration because it's for simple plan jaxrs mount.

Reproduction Path

• Check out hippo-testsuite master.
• Add the following at the bottom of the left menus in site/src/main/webapp/WEB-INF/jsp/components/main/leftmenu.jsp:

    <li>
      <hst:link var="restservicesLink" path="/" mount="restservices"/>
      <a href="${restservicesLink}" 
         target="_blank">RESTful Services</a>
    </li>
  

• Build and run hippo-testsuite master.
• Set /hst:hst/hst:hosts/dev-localhost/localhost/hst:root/restservices/@hst:alias to restservices.
• Set /hst:hst/hst:hosts/dev-localhost/localhost/hst:root/restservices/@hst:ismapped to false.
• Add property, setting /hst:hst/hst:hosts/dev-localhost/localhost/hst:root/@hst:mountrestservices to restservices.
• Visit the homepage, http://localhost:8080/site/.
• See how the RESTful Services menu link is generated.

Expectation:

The RESTful Services menu link should be /site/restservices.

Actual Result

The RESTful Services menu link is generated as /site/.