_getDerivedSubItemsHeading(heading) {
    if (!heading.subItemsHeading) return null;
    return {
      key: heading.subItemsHeading.key || '',
      valueType: heading.subItemsHeading.valueType || heading.valueType,
      granularity: heading.subItemsHeading.granularity || heading.granularity,
      displayUnit: heading.subItemsHeading.displayUnit || heading.displayUnit,
      label: '',
    };
  }
Returns a new heading where the values are defined first by `heading.subItemsHeading`,
and secondly by `heading`. If there is no subItemsHeading, returns null, which will
be rendered as an empty column.
@param {LH.Audit.Details.TableColumnHeading} heading
@return {LH.Audit.Details.TableColumnHeading | null}