Grouped HorizontalBarChart should display legend by default

[jaronheard]

Is your feature request related to a problem? Please describe.
Currently, the HorizontalBarChart with the grouped chart will not display a legend unless it is passed an array of dataSeriesLabels.

Example:
The chart shown in this story requires the following dataSeriesLabel in order to display a legend.

dataSeriesLegend: [
  {
    "category": "Poodle",
    "label": "Poodle"
  },
  {
    "category": "Bulldog",
    "label": "Bulldog"
  }
]

Otherwise, the chart will look like this:

dataSeriesLegend: null

Describe the solution you'd like
Should populate the legend based on the values of dataSeriesKey by default.

Describe alternatives you've considered
Could keep as is, but requires extra work from component users.

Additional context
The legend component can be overridden with the legendComponent prop, so there's no need to provide an extra override to hide the legend.