Pie items in legend groups show up as black squares when hidden

[nicolaskruchten]

Try clicking on either of the trace legend items here: https://codepen.io/nicolaskruchten/pen/LYyPvMq

[nicolaskruchten]

Note that we downscoped #5765 to exclude pie-containing legend groups because of this issue, so when we fix it, we'll want to finish the swing on clickable legend group titles also.