Nil IP addresses options are displayed as "<nil>" in the help message

I just noticed that `docker --help` displays `<nil>` for options (flags) that have a type of `IPVar' and an empty default:

``` shell
% docker --help | grep nil
  --default-gateway=<nil>              Container default gateway IPv4 address
  --default-gateway-v6=<nil>           Container default gateway IPv6 address
```

I think it would be better not to display anything.
