Unexpected serialisation result of empty time series

Given an empty time series instance

```
const emptyTimeSeries = new TimeSeries({
name: 'Empty time series',
columns: [ 'time', 'data' ],
points: []
});
```

I would expect to get the list of columns

```
// Expected [ 'data' ]
// Actual []
emptyTimeSeries.columns();
```

and would expect to get the series serialised

```
// Expected "{"name":"Empty time series","utc":true,"columns":["time","data"],"points":[]}"
// Actual undefined
emptyTimeSeries.toString();
```