Issue #3377623 by g089h515r806: add support for Drupal core's Constraints, add isnull

config/schema/field_validation.schema.yml

@@ -111,3 +111,14 @@ field_validation.rule.email_constraint_rule:
     message:
       type: string
       label: 'Message'
+
+field_validation.rule.null_constraint_rule:
+  type: mapping
+  label: 'IsNull constraint validation rule'
+  mapping:
+    validate_mode:
+      type: string
+      label: 'Validate mode'
+    message:
+      type: string
+      label: 'Message'
\ No newline at end of file

src/Plugin/FieldValidationRule/IsNullConstraintFieldValidationRule.php

+<?php
+
+namespace Drupal\field_validation\Plugin\FieldValidationRule;
+
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\field_validation\ConstraintFieldValidationRuleBase;
+use Drupal\field_validation\FieldValidationRuleSetInterface;
+
+/**
+ * Provides funcationality for EmailFieldValidationRule.
+ *
+ * @FieldValidationRule(
+ *   id = "null_constraint_rule",
+ *   label = @Translation("IsNull constraint"),
+ *   description = @Translation("IsNull constraint.")
+ * )
+ */
+class IsNullConstraintFieldValidationRule extends ConstraintFieldValidationRuleBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getConstraintName(): string{
+    return "Null";
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function isPropertyConstraint(): bool{
+    return FALSE;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function defaultConfiguration() {
+    return [
+      'message' => NULL,
+    ] + parent::defaultConfiguration();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildConfigurationForm(array $form, FormStateInterface $form_state) {
+    $form = parent::buildConfigurationForm($form, $form_state);
+
+    //copied from core.
+    $message = 'This value should be null.';
+
+    $form['message'] = [
+      '#type' => 'textfield',
+      '#title' => $this->t('Message'),
+      '#default_value' => $this->configuration['message'] ?? $message,
+      '#maxlength' => 255,
+    ];
+
+    return $form;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitConfigurationForm(array &$form, FormStateInterface $form_state) {
+    parent::submitConfigurationForm($form, $form_state);
+
+    $this->configuration['message'] = $form_state->getValue('message');
+  }
+
+}