doclet may create a link to a missing anchor/id

In a module summary page, the Navbar may contain a link to "SERVICES", even when there are no services. This means there is a broken link on the page.

We can discuss (separately) whether the word should appear in the subnavbar, but if it does appear, it should not be linked when there is nop target to link to.

<ul class="subNavList">
<li>Module:&nbsp;</li>
<li><a href="#module.description">Description</a>&nbsp;|&nbsp;Modules&nbsp;|&nbsp;<a href="#packages.summary">Packages</a>&nbsp;|&nbsp;<a href="#services.summary">Services</a></li>
</ul>