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
public XContentBuilder doXContentBody(XContentBuilder builder, Params params) throws IOException {
        builder.field("type", "Feature").field("geometry", geoJSONGeometry()).startObject("properties").field("complete", isComplete());
        if (includeSorts) {
            builder.field("sort_values", sortVals);
        }
        builder.endObject();
        return builder;
    }
    private double roundDegrees(double degree) {
        return Math.round(degree * SCALE) / SCALE;
    }
    @Override
    public Object getProperty(List<String> path) {
        if (path.isEmpty()) {
            return this;
        } else if (path.size() == 1 && "value".equals(path.get(0))) {
            return line;
        } else {
            throw new IllegalArgumentException("path not supported for [" + getName() + "]: " + path);
        }
    }
    @Override
    public String toString() {
        return Strings.toString(this);
    }
    @Override
    public int hashCode() {
        return Objects.hash(super.hashCode(), Arrays.hashCode(line), Arrays.hashCode(sortVals), complete, includeSorts, sortOrder, size);
    }
    @Override