Running sbatch_experiment.py with `--broker :` breaks Python

When running 

```
python examples/sbatch_experiment.py --broker $BROKER_IP:$BROKER_PORT -n 32
```

without setting `$BROKER_IP` or `$BROKER_PORT`, moolib causes a hard termination:

```
terminate called after throwing an instance of 'std::invalid_argument'
  what():  In createInetSockAddr at ../../src/tensorpipe/tensorpipe/transport/uv/sockaddr.cc:87 ":"
Aborted (core dumped)
```