Remove erroneous check for >0 args that causes an exit.

This seems like an error. If there's ZERO args we want to quit, not when there is > 0 args.

Additionally, it looks like `mustStack()` handles when a stack is not supplied.

This prevented the following from working:

`cx redeploy -s Emission Central`

Also:
- Removed "os" import as it's no longer necessary.
Merge pull request #5 from joshuapinter/fix_redeploy_args_check

Remove erroneous check for >0 args that causes an exit.