When using "enforce_deepest" and "require_entity", we should always show the deepest level even if no children

When using `'enforce_deepest' => TRUE` and `'require_entity' => TRUE`, it is impossible to select an item at the deepest level because the item at the deepest level has no children, and so it is being removed from the list of options. We should check that if the item is at the deepest level it is not removed from the options.