Elasticsearch 'mapper_parsing_exception'

Join field에 null이 들어가면 fail to parse exception이 남.

{
  "error" : {
    "root_cause" : [
      {
        "type" : "mapper_parsing_exception",
        "reason" : "failed to parse"
      }
    ],
    "type" : "mapper_parsing_exception",
    "reason" : "failed to parse",
    "caused_by" : {
      "type" : "illegal_state_exception",
      "reason" : "[null] expected START_OBJECT or VALUE_STRING but was: VALUE_NULL"
    }
  },
  "status" : 400
}