CLI requires `//sys/client_config/default` to be of type "document"

I just had to debug why `//sys/client_config/default` was not applying in my case. I found that it requires type "document" while I had "map_node":
```
Working:
✗ yt get //sys/client_config/default/@type
"document"
✗ yt get //sys/client_config/default
{
"operation_link_template" = "https://my.cluster/mycluster/operations/{operation_id}/details";
}

Not working:
✗ yt get //sys/client_config/default/@type
"map_node"
✗ yt get //sys/client_config/default
{
"operation_link_template" = "https://my.cluster/mycluster/operations/{operation_id}/details";
}
```

It would be nice to lift that requirement or to provide a warning on client side at least, instead of ignoring the config