Pie-chart coloring doesn't work with an empty column name and no legends

See http://embed.plnkr.co/i0qinQNyWMGD0rkASicU/preview

With a chart defined as follows, the pie-chart will use the same color for both data entries. If `legend.show` is set to `true`, the coloring works as expected.

```
{
data: {
columns: [
['data', 30],
['', 120], // NOTE: empty name
],
type : 'pie',
},
color: {
pattern: ["blue", "red"]
},
legend: {
show: false
}
}
```