Function prototypes cannot return nil, even if stated in the docs

[BlobCodes]

Taking a look at the definition of Gtk::TreeListModelCreateModelFunc:

This function can return %NULL to indicate that @item is guaranteed to be a leaf node and will never have children

However, it is defined as GObject::Object -> Gio::ListModel (we cannot return nil)

[hugopl]

I guess we need need to assume that whatever pointer in callbacks are nullable, due to the lack of GIR annotations for that.

[BlobCodes]

I guess we need need to assume that whatever pointer in callbacks are nullable, due to the lack of GIR annotations for that.

I just checked if there's really no annotation for that.. but there actually is one!
PR is #49