Fix implementation of hook_field_is_empty

While developing my own Relation Select widget I discovered an issue with hook_field_is_empty($item, $field).

Your implementation calls the first parameter $items, suggesting that your implementation assumes that a list of items is given. However, a single item is handed to this function. I have replaced the implementation locally with the following, and it has been working fine for months with my widget. Could you check if it also works with the default widget?

function relation_select_field_is_empty($item, $field){
  return empty($item['endpoints']);
}