Dashed lines connect to null values

When you change a line style from normal to dashed, suddenly it starts connecting to missing/null values.
This can be demonstrated in your online examples using the below code and commenting/uncommenting the regions.

```
var chart = c3.generate({
'data': {
'regions': {
'A_pred_value': [{'start': '2015-01-01', 'style': 'dashed'}]
},
'x': 'month_dt',
'rows': [
['month_dt', 'A_value', 'A_pred_value'],
['2015-01-01', 100, 75.0],
['2015-02-01', 200, 150.0],
['2015-03-01', 150, null],
['2015-04-01', 200, null]
],
'type': 'line'
},
'axis': {
'x': {
'tick': {
'values': ['2014-12-01', '2015-01-01', '2015-02-01', '2015-03-01', '2015-04-01', '2015-05-01'],
'format': '%Y-%m-%d'
},
'type': 'timeseries'
}
}
});
```