Fix the VfsFileLookupFunc's return value so that it is nullable

[fbrouille]

According to the VfsFileLookupFunc's description, the return value can be NULL. The current definition in Gio-2.0.gir does not specify that the return value is nullable. Therefore the generated code of Vfs in gtk-rs is incorrect:

        uri_func: Option<Box_<dyn Fn(&Vfs, &str) -> File + 'static>>,
        parse_name_func: Option<Box_<dyn Fn(&Vfs, &str) -> File + 'static>>,

This pull request fixes the definition of the VfsFileLookupFunc, in order to fix the generated code:

        uri_func: Option<Box_<dyn Fn(&Vfs, &str) -> Option<File> + 'static>>,
        parse_name_func: Option<Box_<dyn Fn(&Vfs, &str) -> Option<File> + 'static>>,

[bilelmoussaoui]

The fix should be submitted to GLib imho.

[sdroege]

Yes this should be submitted to GLib: https://gitlab.gnome.org/GNOME/glib/-/issues/new

Once the annotation is fixed there we will get the fix here automatically.