Fix scalar to geo for empty polygon (#846)

Co-authored-by: Kyle Barron <[email protected]>

Author: b4l

rust/geoarrow/src/io/geo/scalar.rs

@@ -43,8 +43,12 @@ pub fn line_string_to_geo<T: CoordNum>(
 /// Convert any Polygon to a [`geo::Polygon`].
 ///
 /// Only the first two dimensions will be kept.
-pub fn polygon_to_geo<T: CoordNum>(polygon: &impl PolygonTrait<T = T>) -> geo::Polygon<T> {
-    let exterior = line_string_to_geo(&polygon.exterior().unwrap());
+pub fn polygon_to_geo<T: CoordNum, P: PolygonTrait<T = T>>(polygon: &P) -> geo::Polygon<T> {
+    let exterior = if let Some(exterior) = polygon.exterior() {
+        line_string_to_geo(&exterior)
+    } else {
+        geo::LineString::new(vec![])
+    };
     let interiors = polygon
         .interiors()
         .map(|interior| line_string_to_geo(&interior))