Box Plot shows null column name when axis channel title is null

For box plots when the axis title is set to null the tooltip doesn’t fall back to the field name because getTitle only checks whether the title is undefined.

Looks to have been introduced by the combo of #6674 & #6689

Sample spec: https://vega.github.io/editor/#/gist/0bdaadbfd7335144108bcd99aef96458/box_plot_null_axis_title.json

[domoritz]

The behavior is expected. The field title should not be null. We forgot to disallow it but will do that soon. Instead of setting the field title, set the axis title to null.

{
...
  "mark": {"type": "boxplot", "ticks": true},
  "encoding": {
    "x": {
      "type": "ordinal",
      "field": "State",
      "axis": {"title": null}
    },
    "y": {
      "type": "quantitative",
      "field": "Cost",
      "axis": {"title": null}
    }
  }
}

[domoritz]

We still need to fix that null isn’t allowed.