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
new SortedDocValuesField("group_id", new BytesRef(String.valueOf(group)))
                        )
                    );
                }
            }
        }, terms -> {
            for (Terms.Bucket bucket : terms.getBuckets()) {
                InternalGeoLine expectedGeoLine = lines.get(bucket.getKeyAsString());
                InternalGeoLine geoLine = bucket.getAggregations().get("_name");
                assertThat(geoLine.length(), equalTo(expectedGeoLine.length()));
                assertThat(geoLine.isComplete(), equalTo(expectedGeoLine.isComplete()));
                for (int i = 0; i < geoLine.sortVals().length; i++) {
                    geoLine.sortVals()[i] = NumericUtils.sortableLongToDouble((long) geoLine.sortVals()[i]);
                }
                assertArrayEquals(expectedGeoLine.sortVals(), geoLine.sortVals(), 0d);
                assertArrayEquals(expectedGeoLine.line(), geoLine.line());
            }
        });
    }
    private void testCase(
        Query query,
        TermsAggregationBuilder aggregationBuilder,
        CheckedConsumer<RandomIndexWriter, IOException> buildIndex,
        Consumer<Terms> verify
    ) throws IOException {
        testCase(query, aggregationBuilder, buildIndex, verify, NumberFieldMapper.NumberType.LONG);
    }
    private void testCase(
        Query query,
        TermsAggregationBuilder aggregationBuilder,