nul string for output, deletes current directory

[robocoder]

Bug report

Running box compile with this config will delete the current directory:

{
    "output": ""
}

Propose it be treated like:

{
    "output": null
}

[robocoder]

Maybe also check to see if output is a directory? i.e., handle case of

{
  "output": "."
}

[theofidry]

Closed by #533 :)