empty strings and nulls in column names causes data offset

When a column name is an empty string or null, looks like it's being skipped, which causes all the data to be shown incorrectly after that (with offset of 1). If column names are not allowed as empty strings or nulls perhaps it would be better to warn / error, since it 'looks' as if the graph is shown correctly, but actually it's not.
Happens on v0.4.10, reproduction code:

```

```
var x = {
bindto: "#a",
axis: { x: {type: "category"} },
data: {
columns: [
["x", "2014-04", "2014-05", "2014-06", "a", "2014-08", "2014-09", "2014-10"],
["a", null, null, null, null, null, null, "50"],
["b", "10", "11", "9", "2", "16", "2", "50"],
["c", null, null, "13", "16", "20", "5", "50"]
],
x: "x"
}
};

c3.generate(x);
x.bindto="#b";
x.data.columns[0][4] = "";
c3.generate(x);
```