Point selection with null values ​​may not be correct

If `selection` is enabled and data contains `null` value, point selection ​​may not be correct.
I can select the null value at 0 on Y axis.


### Code

```js
var chart = c3.generate({
data: {
x: 'x',
columns: [
['x', '2013-01-01', '2013-01-02', '2013-01-03', '2013-01-04', '2013-01-05', '2013-01-06'],
['data1', -130, 200, 100, 400, 150, 250],
['data2', 130, 340, 200, 500, 250, 350],
['data3', null, 500, 450, 700, 600, 500]
],
selection: {
enabled: true
}
},
axis: {
x: {
type: 'timeseries',
tick: {
format: '%Y-%m-%d'
}
}
}
});
```

### Versions:
* C3 version: 0.7.20
* D3 version: 5.8.0
* Browser: Chrome86
* OS: Windows 10