fix(VDataTable): select all checkbox showing when not enabled on mobile

[webdevnerdstuff]

Description

Fixes the issue where the select all checkbox is showing up next to the sortby select on mobile when data-table-select is not present in the columns

Markup:

const appendIcon = computed(() => {
  const showSelectColumn = columns.value.find(column => column.key === 'data-table-select')

  if (showSelectColumn == null) {
    return
  }

  return allSelected.value ? '$checkboxOn' : someSelected.value ? '$checkboxIndeterminate' : '$checkboxOff'
})