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
CheckedConsumer<RandomIndexWriter, IOException> buildIndex,
        Consumer<Terms> verify,
        NumberFieldMapper.NumberType fieldNumberType
    ) throws IOException {
        Directory directory = newDirectory();
        RandomIndexWriter indexWriter = new RandomIndexWriter(random(), directory);
        buildIndex.accept(indexWriter);
        indexWriter.close();
        IndexReader indexReader = DirectoryReader.open(directory);
        IndexSearcher indexSearcher = newIndexSearcher(indexReader);
        try {
            MappedFieldType fieldType = new GeoPointFieldMapper.GeoPointFieldType("value_field");
            MappedFieldType groupFieldType = new KeywordFieldMapper.KeywordFieldType("group_id", false, true, Collections.emptyMap());
            MappedFieldType fieldType2 = new NumberFieldMapper.NumberFieldType("sort_field", fieldNumberType);
            Terms terms = searchAndReduce(
                indexSearcher,
                new MatchAllDocsQuery(),
                aggregationBuilder,
                fieldType,
                fieldType2,
                groupFieldType
            );
            verify.accept(terms);
        } finally {
            indexReader.close();
            directory.close();
        }
    }
}