fix unknown type fallback serialization of empty feature collection (#286)

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

Author: sargunv

geojson/src/commonMain/kotlin/org/maplibre/spatialk/geojson/GeoJson.kt

@@ -52,7 +52,9 @@ public data object GeoJson {
                         is FeatureCollection<*, *> ->
                             FeatureCollection.serializer(
                                 Geometry.serializer().nullable,
-                                when (val props = obj.features.firstNotNullOf { it.properties }) {
+                                when (
+                                    val props = obj.features.firstNotNullOfOrNull { it.properties }
+                                ) {
                                     is JsonObject? -> JsonObject.serializer().nullable
                                     else -> {
                                         @OptIn(InternalSerializationApi::class)

geojson/src/commonTest/kotlin/org/maplibre/spatialk/geojson/FeatureCollectionTest.kt

@@ -404,6 +404,20 @@ class FeatureCollectionTest {
         }
     }
 
+    @Test
+    fun emptyStarProjection() {
+        // should fall back to JsonObject properties
+        val features: FeatureCollection<*, *> = featureCollectionOf()
+        val json =
+            """
+                {
+                  "type": "FeatureCollection",
+                  "features": []
+                }
+            """
+        assertJsonEquals(json, features.toJson())
+    }
+
     companion object {
         private const val SAMPLE_FEATURECOLLECTION = "sample-featurecollection.json"
         private const val SAMPLE_FEATURECOLLECTION_BBOX = "sample-feature-collection-with-bbox.json"