Donut chart is not closed.

* **C3 version**: 0.7.18
* **D3 version**: the same C3 uses
* **Browser**: Chrome Version 84.0.4147.89
* **OS**: MacOS Mojave

There is an issue with donut chart.
It becomes not closed if some items in the data are zero and positive values, and zero one comes after positive.

Config that is working as expected (no white lines)
`var chart = c3.generate({
data: {
columns: [
['data1', 0],
['data2', 30],
],
type : 'donut',
},

});`

Config that is **NOT** working as expected (white lines)
`
var chart = c3.generate({
data: {
columns: [
['data1', 30],
['data2', 0],
],
type : 'donut',
},

});
`

Please let me know if any additional information needed.