Beware of Drupal's Radios FAPI empty handling

Did you ever switch a Drupal Form API element from "select" to "radios" and expect it would behave equally? You're wrong! Here's (sadly) why... In our drowl_paragraphs module we wanted to switch a setting from "select" to "radios" to allow style preview:

 'select', '#title' => $this->t('Box style'),
 '#options' => $style_boxstyle_options,
 '#empty_option' => $this->t('- None -'),
 '#default_value' => isset($item->style_boxstyle) ? $item->style_boxstyle : '', 
 '#description' => $this->t('Predefined styling of this container.'), ]; ?>

What I expected was, that it's a simple switch and both would behave indentially regarding their empty value. But while a "select" automatically adds a "None" option if

 FALSE

to make it de-selectable, a radio doesn't add such an option. It even doesn't allow for

 $this->t('- None -') // or '#empty_value' => ''

or similar. It simply has all radios unchecked by default and if you've checked a radio, you'll be lost for the future. So the result of my research which lead to to some interesting information here: https://www.drupal.org/project/drupal/issues/1381140 is: If you switch the Drupal Form Element "select" to "radios" you have to add the empty option yourself and it seems best to use an empty string as key and '#default_value':

 $this->t('- None -') ]; 
 
 // [...] $element['style']['style_boxstyle']['style_boxstyle'] = [
 '#type' => 'radios',
 '#title' => $this->t('Box style'),
 '#options' => $style_boxstyle_options,
 '#required' => FALSE,
 '#default_value' => !empty($item->style_boxstyle) ? $item->style_boxstyle : '',
 '#description' => $this->t('Predefined styling of this container.'),
 ];

To automatically add the empty option for select's you may want to add:

See https://api.drupal.org/api/drupal/core!lib!Drupal!Core!Render!Element!S…