treat empty geometries correctly

src/c3nav/mapdata/fields.py

@@ -106,6 +106,8 @@ class GeometryField(models.TextField):
         if value is None:
             return None
         self._validate_geomtype(value, exception=TypeError)
+        if value.is_empty:
+            raise Exception('Cannot save empty geometry.')
         return json.dumps(self.get_final_value(value, as_json=True))
 
     def value_to_string(self, obj):

src/c3nav/mapdata/utils/geometry.py

@@ -41,6 +41,8 @@ class WrappedGeometry():
 
     @cached_property
     def wrapped_geom(self):
+        if not self.wrapped_geojson['coordinates']:
+            return GeometryCollection()
         return shapely_shape(self.wrapped_geojson)
 
     def __getattr__(self, name):