Fix empty geojson failing with style_function

folium/features.py

@@ -579,6 +579,11 @@ def _validate_function(self, func, name):
         Tests `self.style_function` and `self.highlight_function` to ensure
         they are functions returning dictionaries.
         """
+        # If for some reason there are no features (e.g., empty API response)
+        # don't attempt validation
+        if not self.data['features']:
+            return
+
         test_feature = self.data['features'][0]
         if not callable(func) or not isinstance(func(test_feature), dict):
             raise ValueError('{} should be a function that accepts items from '
@@ -629,7 +634,8 @@ def _get_self_bounds(self):
 
     def render(self, **kwargs):
         self.parent_map = get_obj_in_upper_tree(self, Map)
-        if self.style or self.highlight:
+        # Need at least one feature, otherwise style mapping fails
+        if (self.style or self.highlight) and self.data['features']:
             mapper = GeoJsonStyleMapper(self.data, self.feature_identifier,
                                         self)
             if self.style:

tests/test_features.py

@@ -154,6 +154,7 @@ def test_vegalite_major_version(vegalite_spec, version):
     else:
         assert vegalite.vegalite_major_version == version
 
+
 # GeoJsonTooltip GeometryCollection
 def test_geojson_tooltip():
     m = folium.Map([30.5, -97.5], zoom_start=10)
@@ -266,6 +267,14 @@ def _assert_id_got_added(data):
     assert geojson.data['features'][0]['id'] == '0'
 
 
+def test_geojson_empty_features_with_styling():
+    # test we don't fail style function validation when there are no features
+    m = Map()
+    data = {"type": "FeatureCollection", "features": []}
+    GeoJson(data, style_function=lambda x: {}).add_to(m)
+    m.get_root().render()
+
+
 def test_geometry_collection_get_bounds():
     """Assert #1599 is fixed"""
     geojson_data = {