Test doc field existence in elasticsearch facet script

script.sh

curl -XDELETE "http://localhost:9200/script/"

curl -XPUT "http://localhost:9200/script/doc/1" -d'
{
    "title":"test"
}'

curl -XPUT "http://localhost:9200/script/doc/2?refresh" -d'
{
    "title":"nothing",
    "test":"value"
}'

curl -XPOST "http://localhost:9200/script/_search?size=0&pretty" -d'
{
   "facets": {
      "mime_type": {
         "terms": {
            "script": "doc[\"test\"].empty ? null : _source.test"
         }
      }
   }
}'