KDirModel: Consider invalid roots are local fs
If the item set on a node (typically the root node) was null it was implicitly considered local fs. But if the item was not null and the url invalid, it'd be considered network fs.
In the context of KDirModel caching of the fs type guessing this would mean we'd wrongly assume the whole tree is network fs even if it's not.
That's why we change this to consider non-null items with invalid URLs as local fs.