No way to unselect, when max_choices is 1 and field is not required

If you set the cardinality (max_choices) to 1 and do not make the field required, the widget generates a tree with radio buttons. However, once any radio button is selected, you can choose another radio button but you can't unselect completely. This effectively makes the field required.

In the Drupal 7 versions, there would be an "N/A" option. This should be reimplemented for the 2.x branch. (Any tips on where in the code this would be added?)