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
this.line = line;
        this.sortVals = sortVals;
        this.complete = complete;
        this.includeSorts = includeSorts;
        this.sortOrder = sortOrder;
        this.size = size;
    }
    /**
     * Read from a stream.
     */
    public InternalGeoLine(StreamInput in) throws IOException {
        super(in);
        this.line = in.readLongArray();
        this.sortVals = in.readDoubleArray();
        this.complete = in.readBoolean();
        this.includeSorts = in.readBoolean();
        this.sortOrder = SortOrder.readFromStream(in);
        this.size = in.readVInt();
    }
    @Override
    protected void doWriteTo(StreamOutput out) throws IOException {
        out.writeLongArray(line);
        out.writeDoubleArray(sortVals);
        out.writeBoolean(complete);
        out.writeBoolean(includeSorts);
        sortOrder.writeTo(out);
        out.writeVInt(size);
    }
    @Override
    public InternalAggregation reduce(List<InternalAggregation> aggregations, AggregationReduceContext reduceContext) {