Auto-hide the toolbar when passed nil or false.

passing `[]` to `set_toolbar_items` will show an empty toolbar, but passing `nil` or `false` will hide the toolbar if it's being shown.
I think I'd prefer this flavor:

```ruby
def set_toolbar_items(buttons = [], animated = true)
  if buttons
    self.toolbarItems = Array(buttons).map{|b| b.is_a?(UIBarButtonItem) ? b : create_toolbar_button(b) }
    navigationController.setToolbarHidden(false, animated:animated)
  else
    navigationController.setToolbarHidden(true, animated:animated)
  end
end
```