`sandbox.cwd` should always return a valid path

[Vasek Mlejnsky]

Currently sandbox.cwd can return string or none/undefined but that don't make sense because even if a developer doesn't set cwd manually, all operations are happening in the / path.

So by default sandbox.cwd should return /.

[Tomas Valenta]

The new beta SDK does not have the .cwd on the sandbox because of subtle bugs (like this) and because we wanted it as stateless as possible.