Null points render & cause cursor change on mouseover

Putting the following code into https://c3js.org/samples/chart_scatter.html:

```
var chart = c3.generate({
data: {
columns: [
['data1', 30, 200, 100, 400, 150, 250],
['data2', 50, null, 10, 40, 15, 25]
],
type: 'scatter'
}
});
```

At y=0 and x=1, there is a ghost point corresponding to the `null`, which is rendered with `opacity: 0`, but nevertheless causes a cursor change on mouseover.

I'm currently working around this by setting `display: none`. Using d3 `remove()` instead causes some flickering re-rendering issues.

* **C3 version**: 0.7.15
* **D3 version**: 5.8.2
* **Browser**: Chrome 80.0.3987.116
* **OS**: OS X 10.15.2