Hovertemplate should allow nil values from arrays

[alexcjohnson]

There can be good reasons to include nil values (null, Number("NaN"), etc) in your data, but if these are displayed via hovertemplate we act like that field of the template was invalid:

https://codepen.io/alexcjohnson/pen/ogvBpNB?editors=0010

var data = [{
  x: [1,2,3,4],
  y: [1,3,2,3],
  customdata: [[1,2],[3,4],[5,6],[7,null]],
  hovertemplate: "%{customdata[0]} - %{customdata[1]}"
}];

var layout = {
  width: 400, height: 400,
};

Plotly.newPlot(gd, data, layout);

This is confusing to the end viewer, and ugly. Instead, if the data array we pulled the value from explicitly had such an entry we should stringify it (the above would be "null" and "NaN" respectively) and display that. We should only display the un-templated value if you ask for an unrecognized item (%{typo}), or an index that the array doesn't have (%{customdata[2]} when the array only has 2 items for each point))

I'll be glad to make a PR for this.