Make sure wiki module API always return a descriptor for main wiki

It's possible to access the main wiki even when there is no descriptor document but in the code it makes it hard to handle the main wiki is a wiki that exist but does not have a descriptor sometimes.

The best is to return a "virtual" descriptor for main wiki in the API when there is no descriptor document. That way it's always listed as existing wiki and you can always modify and save its descriptor without wondering first if there is really a descriptor already for that wiki.