Default N/A option removal for Boolean fields

Closes #3461121

Added a new variable in field_type in OptionsWidgetBase.php to determine the field type. Based on the field type the adding N/A(empty option) is restricted at OptionsButtonWidget.php

core/lib/Drupal/Core/Field/Plugin/Field/FieldWidget/OptionsButtonsWidget.php

@@ -63,7 +63,7 @@ public function formElement(FieldItemListInterface $items, $delta, array $elemen
    * {@inheritdoc}
    */
   protected function getEmptyLabel() {
-    if (!$this->required && !$this->multiple) {
+    if (!$this->required && !$this->multiple && ($this->field_type != 'boolean')) {
       return $this->t('N/A');
     }
   }