When all values are zero, the y padding is not taken into account

While testing this, I obtain a curve at the bottom of graph without any padding:

```
var chart = c3.generate({
data: {
columns: [
['data1', 0, 0, 0, 0, 0, 0],
],
},
axis: {
y: {
padding: {top: 200, bottom: 200}
},
}
});
```


Reproduce the issue with the c3.generate configuration shown, using six zero values and y-axis top and bottom padding of 200. Trace how the y scale handles an all-zero series and verify that the resulting chart visibly includes the requested padding rather than placing the curve at the bottom.

Written by the indexing model from the issue text.