FlatGeobuf writing: in SPATIAL_INDEX=NO mode, deal with empty geometr…

…ies as if there were null

- this fixes the crash when writing an empty polygon. Fixes OSGeo#11419
- this avoids a reading error when writing an empty line

autotest/ogr/ogr_flatgeobuf.py

@@ -1612,3 +1612,24 @@ def test_ogr_flatgeobuf_arrow_stream_numpy_datetime_as_string(tmp_vsimem):
         assert batch["datetime"][1] == b"2022-05-31T12:34:56.789Z"
         assert batch["datetime"][2] == b"2022-05-31T12:34:56"
         assert batch["datetime"][3] == b"2022-05-31T12:34:56+12:30"
+
+
+def test_ogr_flatgeobuf_write_empty_geometries_no_spatial_index(tmp_vsimem):
+    # Verify empty geom handling without spatial index
+    out_filename = str(tmp_vsimem / "out.fgb")
+    with ogr.GetDriverByName("FlatGeobuf").CreateDataSource(out_filename) as ds:
+        lyr = ds.CreateLayer(
+            "test", geom_type=ogr.wkbPolygon, options=["SPATIAL_INDEX=NO"]
+        )
+
+        f = ogr.Feature(lyr.GetLayerDefn())
+        f.SetGeometry(ogr.CreateGeometryFromWkt("POLYGON EMPTY"))
+        lyr.CreateFeature(f)
+        f = ogr.Feature(lyr.GetLayerDefn())
+        lyr.CreateFeature(f)
+
+    with gdal.OpenEx(out_filename) as ds:
+        lyr = ds.GetLayer(0)
+        f = lyr.GetNextFeature()
+        g = f.GetGeometryRef()
+        assert g is None

ogr/ogrsf_frmts/flatgeobuf/ogrflatgeobuflayer.cpp

@@ -2295,7 +2295,7 @@ OGRErr OGRFlatGeobufLayer::ICreateFeature(OGRFeature *poNewFeature)
         // the size of the FlatBuffer, but WKB might be a good approximation.
         // Takes an extra security margin of 10%
         flatbuffers::Offset<FlatGeobuf::Geometry> geometryOffset = 0;
-        if (ogrGeometry != nullptr)
+        if (ogrGeometry && !ogrGeometry->IsEmpty())
         {
             const auto nWKBSize = ogrGeometry->WkbSize();
             if (nWKBSize > feature_max_buffer_size - nWKBSize / 10)