Describe the bug
The legend doesn't reflect the naming for a splitted series chart if the series group value is 0.

To Reproduce
Using the DataGenerator to create 4 series chart.

const dg = new DataGenerator();
const data1 = dg.generateGroupedSeries(20, 4);
<Chart size={[undefined, 200]}>
  <Settings
    legendPosition={Position.Right}
    showLegendDisplayValue={false}
  />
  <BarSeries
    id={getSpecId('status')}
    name={'Quantity'}
    data={data1}
    xAccessor={'x'}
    yAccessors={['y']}
    splitSeriesAccessors={['g']}
    stackAccessors={['g']}
  />
  <Axis
    id={getAxisId('bottom-axis')}
    position={Position.Bottom}
  />
  <Axis
    id={getAxisId('left-axis')}
    position={Position.Left}
  />
</Chart>

Expected behavior
The legend should reflect the group values. In this case the legend should be 0, 1, 2, 3.

Screenshots

Version (please complete the following information):

• OS: all
• Browser: chrome
• Elastic Charts: master

Additional context
n/a

Errors in browser console
n/a

Kibana Cross Issues
n/a

Checklist

• every related Kibana issue is listed under Kibana Cross Issues list
• kibana cross issue tag is associated to the issue if any kibana cross issue is present