Describe the feature:

Elasticsearch version (bin/elasticsearch --version): 6.3

Plugins installed: []

JVM version (java -version): 1.8

Description of the problem including expected versus actual behavior:
https://discuss.elastic.co/t/aggregation-using-execution-hit-map-automatically-filtered-empty-string-key/95445/2
Issue is same as mentioned in above discuss post.
if I index a doc with empty string {"email": ""}, it's not considered in terms agg response when execution_hint: "map".

Steps to reproduce:

PUT test/_mapping/test
{
  "properties": {
    "email": {
      "type": "keyword"
    }
  }
}

POST test/test/
{
    "email": ""
}

POST test/_search?size=0
{
   "aggregations": {
      "aggregation_name": {
         "terms": {
            "field": "email",
            "execution_hint": "map",
            "missing": "N/A"
         }
      }
   }
}

I expect the behavior to not change based on what execution_hint I am using.
But, maybe the underlying implementation prevents returning the "" bucket?
Are there any workarounds?
Thanks.