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/main/java/org/elasticsearch/xpack/spatial/search/aggregations/InternalGeoLine.java
private long[] line;
    private double[] sortVals;
    private boolean complete;
    private boolean includeSorts;
    private SortOrder sortOrder;
    private int size;
    /**
     * A geo_line representing the bucket for a {@link GeoLineAggregationBuilder}. The values of <code>line</code> and <code>sortVals</code>
     * are expected to be sorted using <code>sortOrder</code>.
     *
     * @param name            the name of the aggregation
     * @param line            the ordered geo-points representing the line
     * @param sortVals        the ordered sort-values associated with the points in the line (e.g. timestamp)
     * @param metadata        the aggregation's metadata
     * @param complete        true iff the line is representative of all the points that fall within the bucket. False otherwise.
     * @param includeSorts    true iff the sort-values should be rendered in xContent as properties of the line-string. False otherwise.
     * @param sortOrder       the {@link SortOrder} for the line. Whether the points are to be plotted in asc or desc order
     * @param size            the max length of the line-string.
     */
    InternalGeoLine(
        String name,
        long[] line,
        double[] sortVals,
        Map<String, Object> metadata,
        boolean complete,
        boolean includeSorts,
        SortOrder sortOrder,
        int size
    ) {
        super(name, metadata);