Add docs and a codepen explaining null vs. undefined in restyle

[rreusser]

This PR adds docs and a codepen explaining the behavior of null vs. undefined. (null affirmatively resets to default, undefined is understood as unspecified and is ignored.) It's a subtle-but-useful feature that was added in plotly/plotly.js#844

[cldougl]

nice @rreusser !!
I think it may be helpful to add a null/null ("reset") button in so that users can see how each option affects the graph from the beginning state.

[rreusser]

Good idea. Updated!

[cldougl]

💃 💃