Allow null from/to in range aggregation

[pquentin]

This fixes the following YAML test in the Search API:

{
  "api": "search",
  "file": "/test/free/aggregations/range.yml",
  "name": "Null to and from",
  "origin": "yaml",
  "request": {
    "args": {
      "body": {
        "aggs": {
          "double_range": {
            "range": {
              "field": "double",
              "ranges": [
                {
                  "from": null,
                  "to": 50
                },
                {
                  "from": 50,
                  "to": 150
                },
                {
                  "from": 150,
                  "to": null
                }
              ]
            }
          }
        },
        "size": 0
      },
      "index": "test",
      "typed_keys": true
    }
  },
  "response": {
    "headers": {
      "content-encoding": "gzip",
      "content-type": "application/json",
      "transfer-encoding": "chunked",
      "x-elastic-product": "Elasticsearch"
    },
    "payload": {
      "_shards": {
        "failed": 0,
        "skipped": 0,
        "successful": 1,
        "total": 1
      },
      "aggregations": {
        "range#double_range": {
          "buckets": [
            {
              "doc_count": 1,
              "key": "*-50.0",
              "to": 50
            },
            {
              "doc_count": 2,
              "from": 50,
              "key": "50.0-150.0",
              "to": 150
            },
            {
              "doc_count": 0,
              "from": 150,
              "key": "150.0-*"
            }
          ]
        }
      },
      "hits": {
        "hits": [
        ],
        "max_score": null,
        "total": {
          "relation": "eq",
          "value": 4
        }
      },
      "timed_out": false,
      "took": 2
    },
    "statusCode": 200
  }
}

[flobernd]

LGTM as well!