"Unparseable number" exception when missing is used with format in terms agg

[imotov]

Reproduces with both 7.x and master:

DELETE test?ignore_unavailable=true

POST test/_bulk?refresh
{"index":{}}
{"val": 1}


POST test/_search
{
  "size": 10, 
  "aggs": {
    "test": {
      "terms": {
        "field": "val",
        "format": "test ###.0#",
        "size": 10,
        "missing": 42
      }
    }
  }
}

The error:

{
  "error" : {
    "root_cause" : [
      {
        "type" : "parse_exception",
        "reason" : "parse_exception: Unparseable number: \"42\""
      }
    ],
    "type" : "search_phase_execution_exception",
    "reason" : "all shards failed",
    "phase" : "query",
    "grouped" : true,
    "failed_shards" : [
      {
        "shard" : 0,
        "index" : "test",
        "node" : "0inRi1i0RIiutV9RuJiNlg",
        "reason" : {
          "type" : "runtime_exception",
          "reason" : "runtime_exception: java.text.ParseException: Unparseable number: \"42\"",
          "caused_by" : {
            "type" : "parse_exception",
            "reason" : "parse_exception: Unparseable number: \"42\""
          }
        }
      }
    ],
    "caused_by" : {
      "type" : "parse_exception",
      "reason" : "parse_exception: Unparseable number: \"42\""
    }
  },
  "status" : 500
}

[elasticmachine]

Pinging @elastic/es-analytics-geo (Team:Analytics)