Resolve undefined environment variables to the empty string in the cli.

When using a manifest that references undefined environment variables for port values, the pack fails to start because `undefined` is not a valid port parameter for a `Server`. This change causes the cli to treat undefined environment variables as the empty string. In the case where a port value is parsed, this will cause the server to use an ephemeral port.
Oops, sorry about that. Fixed.

Have you considered using a tool like [jscs](https://github.com/jscs-dev/node-jscs) to automate some of this?