Geoline aggregator might throw an AIOOBE if parent aggregation emits empty buckets

[iverase]

to reproduce the issue:

create two indexes:

PUT /test1
{
  "mappings": {
    "properties": {
      "location": {
        "type": "geo_point"
      },
      "date": {
        "type": "date"
      },
      "entity": {
        "type": "keyword"
      }
    }
  }
}

PUT /test2
{
  "mappings": {
    "properties": {
      "location": {
        "type": "geo_point"
      },
      "date": {
        "type": "date"
      },
      "entity": {
        "type": "keyword"
      }
    }
  }
}

Insert some data:

POST /_bulk
{ "index" : { "_id" : "1", "_index" : "test2" } }
{ "date" : "2020-01-01T01:00:00.0Z", "entity" : "e1", "location" : { "lat" : 50.3, "lon" : 0.13 } } 
{ "index" : { "_id" : "1", "_index" : "test2" } }
{ "date" : "2020-01-01T01:00:00.0Z", "entity" : "e2", "location" : { "lat" : 50.3, "lon" : 0.13 } } 

execute the following query:

GET test1,test2/_search
{
  "size": 0,
  "aggs": {
    "tracks": {
      "filters": {
        "filters": {
          "1": {
            "term": {
              "entity": "e1"
            }
          },
          "2": {
            "term": {
              "entity": "e2"
            }
          }
        }
      },
      "aggs": {
        "path": {
          "geo_line": {
            "point": {
              "field": "location"
            },
            "sort": {
              "field": "date"
            }
          }
        }
      }
    }
  }
}

you would see something like:

 {
        "shard" : 0,
        "index" : "test1",
        "node" : "oOjCv2xpQmm-rWmAo0mcZg",
        "reason" : {
          "type" : "array_index_out_of_bounds_exception",
          "reason" : "Index 1 out of bounds for length 1"
        }
      }

[elasticmachine]

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