Merge pull request #1785 from yoonghm/patch-3

Update _chart_types.py

Author: nicolaskruchten

Diff for: packages/python/plotly/plotly/express/_chart_types.py

@@ -779,7 +779,7 @@ def bar_polar(
     labels={},
     color_discrete_sequence=None,
     color_discrete_map={},
-    barnorm="",
+    barnorm=None,
     barmode="relative",
     direction="clockwise",
     start_angle=90,