Empty response contains one empty row, should contain none

[jheeffer]

https://big-waffle.gapminder.org/sg-master/d90f64d?_select_key@=country;&value@=world/_4region;;&from=entities&where_$and@_un/_state:true;&_world/_4region=africa

This response should not contain the inner empty array in rows

{
  "version": "d90f64d",
  "header": [
    "country",
    "world_4region"
  ],
  "rows": [

  ],
  "info": [
    {
      "msg": "DB found 0 records",
      "time": "2021-04-03T13:29:55.756Z"
    }
  ]
}

This should also be reflected in vizabi-ddfservice-reader, which now returns a response with one empty row.