Replace !Entity::has() with Entity::isEmpty()

- Fix issue where fields with null values triggered errors in buildRules.
- Refer: #63

src/Model/Behavior/EnumBehavior.php

@@ -292,7 +292,7 @@ public function __call(string $method, array $args): bool
             throw new MissingEnumConfigurationException([$alias]);
         }
 
-        if (!$entity->has($config['field']) && Hash::get($config, 'allowEmpty') === true) {
+        if ($entity->isEmpty($config['field']) && Hash::get($config, 'allowEmpty') === true) {
             return true;
         }