Fix `None` within `reactpy.html`

[Archmonger]

Current Situation

Currently None within reactpy.html elements get rendered as the text literal None, rather than doing nothing.

Tangentially related:

• https://github.com/reactive-python/reactpy/issues/538
• https://github.com/reactive-python/reactpy/issues/459

Proposed Actions

We should treat None as a positional placeholder that does nothing. We need to make sure that it doesn't mess up how we auto-generate positional keys.

If we want to handle this server-side, we can substitute it with html._ as a quick-n-easy hack.