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
import org.elasticsearch.plugins.SearchPlugin;
import org.elasticsearch.search.aggregations.AggregatorTestCase;
import org.elasticsearch.search.aggregations.bucket.terms.Terms;
import org.elasticsearch.search.aggregations.bucket.terms.TermsAggregationBuilder;
import org.elasticsearch.search.aggregations.support.MultiValuesSourceFieldConfig;
import org.elasticsearch.search.sort.SortOrder;
import org.elasticsearch.xpack.spatial.SpatialPlugin;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import java.util.Map;
import java.util.function.Consumer;
import static org.hamcrest.CoreMatchers.equalTo;
public class GeoLineAggregatorTests extends AggregatorTestCase {
    @Override
    protected List<SearchPlugin> getSearchPlugins() {
        return Collections.singletonList(new SpatialPlugin());
    }
    
    public void testMixedMissingValues() throws IOException {
        MultiValuesSourceFieldConfig valueConfig = new MultiValuesSourceFieldConfig.Builder().setFieldName("value_field").build();
        MultiValuesSourceFieldConfig sortConfig = new MultiValuesSourceFieldConfig.Builder().setFieldName("sort_field").build();
        GeoLineAggregationBuilder lineAggregationBuilder = new GeoLineAggregationBuilder("_name").point(valueConfig)
            .sortOrder(SortOrder.ASC)
            .sort(sortConfig)
            .size(10);
        TermsAggregationBuilder aggregationBuilder = new TermsAggregationBuilder("_name").field("group_id")
            .subAggregation(lineAggregationBuilder);