Fix a bug in the GeoJSON handling when there are no active polygons

internal/providers/spc/geojson.go

@@ -33,6 +33,11 @@ type GeoJSONProperties struct {
 	DN         *int   `json:"DN"`
 }
 
+type geometryMetadata struct {
+	Type       string            `json:"type"`
+	Geometries []json.RawMessage `json:"geometries"`
+}
+
 // DecodeGeoJSON decodes an SPC GeoJSON outlook product and compacts feature
 // geometry JSON for stable downstream storage.
 func DecodeGeoJSON(raw []byte) (GeoJSONFeatureCollection, error) {
@@ -50,6 +55,16 @@ func DecodeGeoJSON(raw []byte) (GeoJSONFeatureCollection, error) {
 	return collection, nil
 }
 
+// IsEmptyGeometryCollection reports whether raw is SPC's no-polygon placeholder
+// geometry shape: a GeometryCollection with no child geometries.
+func IsEmptyGeometryCollection(raw json.RawMessage) bool {
+	var meta geometryMetadata
+	if err := json.Unmarshal(raw, &meta); err != nil {
+		return false
+	}
+	return meta.Type == "GeometryCollection" && len(meta.Geometries) == 0
+}
+
 func (p *GeoJSONProperties) UnmarshalJSON(raw []byte) error {
 	type alias GeoJSONProperties
 	var aux struct {