Tries to fix #12. Since sources can define for themselves how an empty result can look like, there is a chance the feature will break again, since a failing empty result check is the cause of the problem. The code now supports the following 3 result variants:

{
  [1] = {},
  [2] = {
    result = {}
  },
  [3] = {
    result = {
      <proper result>
    }
  }
}