Do not add newlines to empty strings

[niknetniko]

When updating the documentation, it occurred to me that we currently also append newlines to empty strings for stdout/stderr. This is probably not what we want (since an empty stdout/stderr is perfectly fine), and also not how POSIX defines a file (zero or more lines).

[pdawyndt]

I actually ran into this issue for one of the JavaScript exercises: after changing the newline handling, there was a single test suite that had expected stderr specified as the empty string. I had to remove the specification of stderr (which was actually not needed) for exactly the reason that was mentioned here. The fix would have indeed resolved the issue as well.