feat: update default values

src/Appwrite/Utopia/Response/Model/Key.php

@@ -59,9 +59,9 @@ class Key extends Model
                 'example' => '919c2d18fb5d4...a2ae413da83346ad2',
             ])
             ->addRule('accessedAt', [
-                'type' => self::TYPE_DATETIME_EXAMPLE,
+                'type' => self::TYPE_DATETIME,
                 'description' => 'Most recent access date in Unix timestamp.',
-                'default' => null,
+                'default' => '',
                 'example' => self::TYPE_DATETIME_EXAMPLE
             ])
             ->addRule('sdks', [

src/Appwrite/Utopia/Response/Model/User.php

@@ -65,7 +65,7 @@ class User extends Model
             ->addRule('registration', [
                 'type' => self::TYPE_DATETIME,
                 'description' => 'User registration date in Datetime.',
-                'default' => null,
+                'default' => '',
                 'example' => self::TYPE_DATETIME_EXAMPLE,
             ])
             ->addRule('status', [