Fix None check in update_chart to work with DataFrames

What we have now will fail if you submit a DataFrame, saying:


ValueError: The truth value of a DataFrame is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().

datawrapper/__main__.py

@@ -809,7 +809,7 @@ def update_chart(
             obj = self.get_chart(chart_id)
 
         # Add data, if provided
-        if data:
+        if data is not None:
             self.add_data(chart_id=obj["id"], data=data)
 
         # Return the result