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
int mergedSize = 0;
        boolean reducedComplete = true;
        boolean reducedIncludeSorts = true;
        List<InternalGeoLine> internalGeoLines = new ArrayList<>(aggregations.size());
        for (InternalAggregation aggregation : aggregations) {
            InternalGeoLine geoLine = (InternalGeoLine) aggregation;
            internalGeoLines.add(geoLine);
            mergedSize += geoLine.line.length;
            reducedComplete &= geoLine.complete;
            reducedIncludeSorts &= geoLine.includeSorts;
        }
        reducedComplete &= mergedSize <= size;
        int finalSize = Math.min(mergedSize, size);
        MergedGeoLines mergedGeoLines = new MergedGeoLines(internalGeoLines, finalSize, sortOrder);
        mergedGeoLines.merge();
        
        if (reduceContext.isFinalReduce() && SortOrder.DESC.equals(sortOrder)) {
            new PathArraySorter(mergedGeoLines.getFinalPoints(), mergedGeoLines.getFinalSortValues(), SortOrder.ASC).sort();
        }
        return new InternalGeoLine(
            name,
            mergedGeoLines.getFinalPoints(),
            mergedGeoLines.getFinalSortValues(),
            getMetadata(),
            reducedComplete,
            reducedIncludeSorts,
            sortOrder,
            size
        );
    }