Fix host on gem push, when not passing a --host

After d3f91a011705020c4f1e601d8ddab1c924e5eb99 we start accepting the option --host on the push command.
However when the --host is not provided, it is set as nil, and this was causing the following output:
```
Enter your  credentials.
Dont have an account yet? Create one at https:///sign_up
```

With the patch, on `sign_in`, we get the host even if a nil one is provided.