Reproduction (from the docs, but removed 2 of the 3 docs):

```
PUT test
{
    "mappings": {
        "dynamic": "strict",
        "_source": {
            "enabled": false
        },
        "properties": {
            "my_location": {
                "type": "geo_point"
            },
            "group": {
                "type": "keyword"
            },
            "@timestamp": {
                "type": "date"
            }
        }
    }
}

POST /test/_bulk?refresh
{"index": {}}
{"my_location": {"lat":37.3450570, "lon": -122.0499820}, "@timestamp": "2013-09-06T16:00:36"}

POST /test/_search?filter_path=aggregations
{
  "aggs": {
    "line": {
      "geo_line": {
        "point": {"field": "my_location"},
        "sort": {"field": "@timestamp"}
      }
    }
  }
}
```

This reports:

```
      "geometry" : {
        "coordinates" : [
          [
            -122.049982,
            37.345057
          ]
        ],
        "type" : "LineString"
      },
```

which is invalid as a line should have at least 2 points, e.g. elasticsearch errors if you try to index a line with just 1 point.

If you use the geo-line aggregation in a transform, transform fails if it stumbles upon such a document.

It seems to me that the geo-line aggregation should not report these points or it should report it as points.
x-pack/plugin/spatial/src/test/java/org/elasticsearch/xpack/spatial/search/aggregations/GeoLineAggregatorTests.java
/*
 * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
 * or more contributor license agreements. Licensed under the Elastic License
 * 2.0; you may not use this file except in compliance with the Elastic License
 * 2.0.
 */
package org.elasticsearch.xpack.spatial.search.aggregations;
import org.apache.lucene.document.Field;
import org.apache.lucene.document.LatLonDocValuesField;
import org.apache.lucene.document.SortedDocValuesField;
import org.apache.lucene.document.SortedNumericDocValuesField;
import org.apache.lucene.geo.GeoEncodingUtils;
import org.apache.lucene.index.DirectoryReader;
import org.apache.lucene.index.IndexReader;
import org.apache.lucene.search.IndexSearcher;
import org.apache.lucene.search.MatchAllDocsQuery;
import org.apache.lucene.search.Query;
import org.apache.lucene.store.Directory;
import org.apache.lucene.tests.index.RandomIndexWriter;
import org.apache.lucene.util.BytesRef;
import org.apache.lucene.util.NumericUtils;
import org.elasticsearch.common.util.Maps;
import org.elasticsearch.core.CheckedConsumer;
import org.elasticsearch.geo.GeometryTestUtils;
import org.elasticsearch.geometry.Point;
import org.elasticsearch.index.mapper.GeoPointFieldMapper;
import org.elasticsearch.index.mapper.KeywordFieldMapper;
import org.elasticsearch.index.mapper.MappedFieldType;
import org.elasticsearch.index.mapper.NumberFieldMapper;