[TIMOB-17227] CLI: Passing empty option value, building the project returns Error

For https://jira.appcelerator.org/browse/TIMOB-17227
Instead of logging the error and exiting, instead replace both lines with

    return;

It's sorta complicated, but `callback()` is fired with an option has a value set and `callback()` can return a modified value. An empty string `projectDir` is passed in and we are modifying it by returning `undefined`. Internally this sets `cli.argv['project-dir']` to undefined and the CLI's validation system will proceed to prompt for the project directory.

This is more elegant and consistent. Sorry I didn't think of it sooner.