Node controller doesn't show `None` well

[liamhuber]

Since #131, input can only be modified in the node controller if (a) it's not connected to receive from the graph, and (b) there is an existing widget that handles that data type. In particular, for float data, transitioning from ipywidgets.Text to ipywidgets.TextFloat means that any initial None values no longer register in the widget, the widget default (0 in this case) is shown instead.

As far as I can tell this is just a fundamental limitation with the widgets, which don't have any sort of allow_none flag. I had hoped to at least add the default as a tooltip so when you moused over you could at least read the default, but tooltips aren't even implemented for TextFloat.

So for now let's just live with it.