make sure it gives proper errors when run on whitespace or comment-only strings

[pgbovine]

e.g.,:

• empty string
• a string of whitespaces
• only comments like "# just a comment here" <-- it currently displays something strange
• a legal R program but one that doesn't contain pipes or data frames

[pgbovine]

e.g.,

$ node run-backend.js r '#just a comment   '
{
  "code": "#just a comment   ",
  "trace": [
    {
      "type": "error",
      "code_step": "NA",
      "mapping": {
        "message": {
          "header": "attempt to select less than one element in integerOneIndex",
          "body": [],
          "footer": []
        }
      },
      "data_frame": "NA"
    }
  ]
}

$ node run-backend.js r '   '
{
  "code": "   ",
  "trace": [
    {
      "type": "error",
      "code_step": "NA",
      "mapping": {
        "message": {
          "header": "attempt to select less than one element in integerOneIndex",
          "body": [],
          "footer": []
        }
      },
      "data_frame": "NA"
    }
  ]
}

[seankross]

Fixed in 18edf68