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
.size(size);
        TermsAggregationBuilder aggregationBuilder = new TermsAggregationBuilder("_name").field("group_id")
            .subAggregation(lineAggregationBuilder);
        Map<String, InternalGeoLine> lines = Maps.newMapWithExpectedSize(1);
        String groupOrd = "0";
        long[] points = new long[numPoints];
        double[] sortValues = new double[numPoints];
        for (int i = 0; i < numPoints; i++) {
            Point point = GeometryTestUtils.randomPoint(false);
            int encodedLat = GeoEncodingUtils.encodeLatitude(point.getLat());
            int encodedLon = GeoEncodingUtils.encodeLongitude(point.getLon());
            long lonLat = (((long) encodedLon) << 32) | encodedLat & 0xffffffffL;
            points[i] = lonLat;
            sortValues[i] = i;
        }
        int lineSize = Math.min(numPoints, size);
        
        long[] linePoints = Arrays.copyOf(points, lineSize);
        double[] lineSorts = Arrays.copyOf(sortValues, lineSize);
        new PathArraySorter(linePoints, lineSorts, SortOrder.ASC).sort();
        lines.put(groupOrd, new InternalGeoLine("_name", linePoints, lineSorts, null, complete, true, SortOrder.ASC, size));
        testCase(new MatchAllDocsQuery(), aggregationBuilder, iw -> {
            for (int i = 0; i < points.length; i++) {
                int x = (int) (points[i] >> 32);
                int y = (int) points[i];
                iw.addDocument(
                    Arrays.asList(
                        new LatLonDocValuesField("value_field", GeoEncodingUtils.decodeLatitude(y), GeoEncodingUtils.decodeLongitude(x)),
                        new SortedNumericDocValuesField("sort_field", NumericUtils.doubleToSortableLong(sortValues[i])),
                        new SortedDocValuesField("group_id", new BytesRef(groupOrd))