Make it possible to use an empty string in a select field

src/Form/Control/Callbacks/SelectCallbacks.php

@@ -43,7 +43,7 @@ public static function validate(array $element, FormStateInterface $formState):
    */
   public static function value(array &$element, $input, FormStateInterface $formState) {
     if (array_key_exists('#empty_value', $element) && $input === (string) $element['#empty_value']) {
-      $input = '' === $element['#empty_value'] ? NULL : $element['#empty_value'];
+      $input = $element['#_empty_value'];
 
       if (NULL === $input) {
         // Prevent empty string as value. Drupal sets an empty string in this

src/Form/Control/CheckboxesArrayFactory.php

@@ -26,7 +26,7 @@
 use Drupal\json_forms\Form\AbstractConcreteFormArrayFactory;
 use Drupal\json_forms\Form\Control\Callbacks\CheckboxesCallbacks;
 use Drupal\json_forms\Form\Control\Util\BasicFormPropertiesFactory;
-use Drupal\json_forms\Form\Control\Util\OptionsBuilder;
+use Drupal\json_forms\Form\Control\Util\OptionsUtil;
 use Drupal\json_forms\Form\FormArrayFactoryInterface;
 use Drupal\json_forms\Form\Util\FormCallbackRegistrator;
 use Drupal\json_forms\JsonForms\Definition\Control\ArrayControlDefinition;
@@ -54,7 +54,7 @@ public function createFormArray(
     $form = [
       '#type' => 'checkboxes',
       // @todo Handle non-string values and integerish strings.
-      '#options' => OptionsBuilder::buildOptions($definition),
+      '#options' => OptionsUtil::buildOptions($definition),
     ] + BasicFormPropertiesFactory::createFieldProperties($definition, $formState);
 
     if (isset($form['#default_value'])) {

src/Form/Control/RadiosArrayFactory.php

@@ -26,7 +26,7 @@
 use Drupal\json_forms\Form\AbstractConcreteFormArrayFactory;
 use Drupal\json_forms\Form\Control\Callbacks\OptionValueCallbacks;
 use Drupal\json_forms\Form\Control\Util\BasicFormPropertiesFactory;
-use Drupal\json_forms\Form\Control\Util\OptionsBuilder;
+use Drupal\json_forms\Form\Control\Util\OptionsUtil;
 use Drupal\json_forms\Form\FormArrayFactoryInterface;
 use Drupal\json_forms\JsonForms\Definition\Control\ControlDefinition;
 use Drupal\json_forms\JsonForms\Definition\DefinitionInterface;
@@ -49,8 +49,8 @@ public function createFormArray(
     /** @var \Drupal\json_forms\JsonForms\Definition\Control\ControlDefinition $definition */
     $form = [
       '#type' => 'radios',
-      '#options' => OptionsBuilder::buildOptions($definition),
-      '#_option_values' => OptionsBuilder::buildOptionValues($definition),
+      '#options' => OptionsUtil::buildOptions($definition),
+      '#_option_values' => OptionsUtil::buildOptionValues($definition),
       '#value_callback' => OptionValueCallbacks::class . '::value',
       '#element_validate' => [OptionValueCallbacks::class . '::validate'],
       '#_type' => $definition->getType(),

src/Form/Control/SelectArrayFactory.php

@@ -26,7 +26,7 @@
 use Drupal\json_forms\Form\AbstractConcreteFormArrayFactory;
 use Drupal\json_forms\Form\Control\Callbacks\SelectCallbacks;
 use Drupal\json_forms\Form\Control\Util\BasicFormPropertiesFactory;
-use Drupal\json_forms\Form\Control\Util\OptionsBuilder;
+use Drupal\json_forms\Form\Control\Util\OptionsUtil;
 use Drupal\json_forms\Form\FormArrayFactoryInterface;
 use Drupal\json_forms\JsonForms\Definition\Control\ControlDefinition;
 use Drupal\json_forms\JsonForms\Definition\DefinitionInterface;
@@ -49,15 +49,22 @@ public function createFormArray(
     /** @var \Drupal\json_forms\JsonForms\Definition\Control\ControlDefinition $definition */
     $form = [
       '#type' => 'select',
-      '#options' => OptionsBuilder::buildOptions($definition),
-      '#_option_values' => OptionsBuilder::buildOptionValues($definition),
+      '#options' => OptionsUtil::buildOptions($definition),
+      '#_option_values' => OptionsUtil::buildOptionValues($definition),
       '#value_callback' => SelectCallbacks::class . '::value',
       '#element_validate' => [SelectCallbacks::class . '::validate'],
     ] + BasicFormPropertiesFactory::createFieldProperties($definition, $formState);
 
+    // #empty_value cannot be NULL, thus we use #_empty_value to allow NULL as
+    // well as an empty string.
+    $form['#_empty_value'] = OptionsUtil::getEmptyOptionValue($definition);
     if (!$definition->isRequired()) {
       $form['#empty_value'] = '';
     }
+    // @phpstan-ignore offsetAccess.nonOffsetAccessible
+    if (isset($form['#options']['']) && '' !== $form['#options']['']) {
+      $form['#empty_option'] = $form['#options'][''];
+    }
 
     return $form;
   }

src/Form/Control/Util/OptionsBuilder.php → src/Form/Control/Util/OptionsUtil.php

@@ -24,7 +24,7 @@
 use Drupal\json_forms\JsonForms\Definition\Control\ArrayControlDefinition;
 use Drupal\json_forms\JsonForms\Definition\Control\ControlDefinition;
 
-final class OptionsBuilder {
+final class OptionsUtil {
 
   /**
    * @param \Drupal\json_forms\JsonForms\Definition\Control\ControlDefinition $definition
@@ -73,6 +73,28 @@ public static function buildOptionValues(ControlDefinition $definition): array {
     return $optionValues;
   }
 
+  /**
+   * @return ''|null
+   *   The actual value to use if the option with the empty value is selected.
+   */
+  public static function getEmptyOptionValue(ControlDefinition $definition): ?string {
+    $enums = self::getEnum($definition);
+    if (in_array('', $enums, TRUE)) {
+      return '';
+    }
+    if (in_array(NULL, $enums, TRUE)) {
+      return NULL;
+    }
+
+    foreach (self::getOneOf($definition) as $option) {
+      if (\property_exists($option, 'const') && (NULL === $option->const || '' === $option->const)) {
+        return $option->const;
+      }
+    }
+
+    return NULL;
+  }
+
   /**
    * @phpstan-return array<scalar|null>
    */