fix(data-table): allow sorting-order to be null

[joshLumpkin]

closes #4063

Description

I added null as a prop option on the sortingOrder

Markup:

export const useSortableProps = {
  ...useThrottleProps,
  sortBy: { type: String as PropType<string | undefined> },
  columnSorted: { type: Object as PropType<any | undefined> },
  sortingOrder: { type: String as PropType<DataTableSortingOrder | null | undefined> },
}

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement/refactoring (non-breaking change that doesn't add any feature but make things better)