Include empty nested fields in aggregation

[Calin_Blaga (Călin Blaga)]

I would like to do an aggregation on a nested field, and I would like to include empty occurrences too.
What I tried:

GET index/_search
{
  "size": 0,
  "aggs": {
    "language_skills": {
      "nested": {
        "path": "language_skills"
      },
      "aggs": {
        "result": {
          "terms": {
            "field": "language_skills.language",
            "missing": "N/A"
          }
        }
      }
    }
  }
}

what I get:

"aggregations" : {
    "language_skills" : {
      "doc_count" : 157,
      "result" : {
        "doc_count_error_upper_bound" : 0,
        "sum_other_doc_count" : 0,
        "buckets" : [
          {
            "key" : "de",
            "doc_count" : 73
          },
          {
            "key" : "en",
            "doc_count" : 62
          },
          {
            "key" : "fr",
            "doc_count" : 19
          },
          {
            "key" : "it",
            "doc_count" : 3
          }
        ]
      }
    }
  }

What I expect:

"aggregations": {
  "language_skills": {
    "doc_count": 157,
    "result": {
      "doc_count_error_upper_bound": 0,
      "sum_other_doc_count": 0,
      "buckets": [
        {
          "key": "de",
          "doc_count": 73
        },
        {
          "key": "en",
          "doc_count": 62
        },
        {
          "key": "fr",
          "doc_count": 19
        },
        {
          "key": "it",
          "doc_count": 3
        },
        {
          "key": "N/A",        <---- this bucket is missing
          "doc_count": 21
        }
      ]
    }
  }
}

Tried some other queries as well, with no luck.
Any help would be highly appreciated.

[Calin_Blaga (Călin Blaga)]

Looks like it has been asked already but no answer How to aggregate nested fields with null values? - #2 by Doronpt

[Calin_Blaga (Călin Blaga)]

Tried this too, but it returns the whole result instead of what is expected:

GET index/_search
{
  "size": 0, 
  "aggs": {
    "language": {
      "nested": {
        "path": "language_skills"
      },
      "aggs": {
        "language": {
          "missing": {
            "field": "language_skills.language"
          }
        }
      }
    }
  }
}