[GOption] pre_parse/post_parse of main group are not called

Following code:

    GOptionGroup *group = g_option_group_new (nullptr, nullptr, nullptr,
                                              closure, nullptr);
    g_option_group_add_entries (group, entries);
    g_option_group_set_parse_hooks (group, pre_parse, post_parse);
    g_option_context_set_main_group (context, group);

the pre_parse/post_parse are not called. Checking the code, they are only called for groups added using g_option_context_add_group.