allow reports without forcing a link in the "project reports" section

this patch allows report plugins to work also without a link in the "project reports" section.
Reports registered without the "link" attribute will be generated but not added to the menu or project reports page.

This can be useful for reports you want to link "manually" into other sections, like for example the FAQ report.
This can be achieved with:
<doc:registerReport
name="FAQs"
pluginName="maven-faq-plugin"
description="Frequently Asked Questions."
/>

Without this patch the link is generated to a "/.html" file.