`render_resource_link` should add the "missing" class if the resource doesn't exist

[Ryan J Ollos <ryan.j.ollos@…>]

In the case that get_resource_description doesn't return an Element, render_resource_link should check whether the resource exists and add the missing class if it doesn't.

This is mainly useful for plugins at the moment, since render_resource_link is only utilized once in the Trac source. In this one case that it is used, a context parameter is passed, so an Element will be returned by render_resource_link and the pathway that this ticket proposes to modify is not being executed in that case.

I've prepared a patch in ​ff9c10ec. I don't like how the test case adds a dependency on trac.wiki.model.WikiPage, but I didn't have any better ideas at the moment and struggled to figure out the right way to code a "dummy" ResourceManager. I'll iterate on the patch if anyone can provide suggestions on how to improve.