Add missing test for nullable properties (could add more)

tests/Entities/EntityTwo.php

@@ -0,0 +1,18 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Edudobay\DoctrineSerializable\Tests\Entities;
+
+use Edudobay\DoctrineSerializable\Attributes\Serializable;
+
+class EntityTwo
+{
+    public function __construct(
+        #[Serializable]
+        public ?string $name
+    ) {
+    }
+
+    public ?string $_name;
+}

tests/SerializationHandlerTest.php

@@ -71,11 +71,23 @@ public function test_can_deserialize_JSON_string_instead_of_array(): void
         self::assertEquals(new Entities\User('mickey42', 'Mickey Mouse'), $e->user);
     }
 
-    public function test_nullable_attributes(): void
+    // TODO: What else can we verify about nullable properties?
+    public function test_null_property_is_serialized_to_PHP_null(): void
     {
-        self::markTestIncomplete('TODO');
+        // 1. Set a non-null value
+        $e = new Entities\EntityTwo(name: 'pencil');
+        $this->handler()->serialize($e);
+        self::assertSame('pencil', $e->_name);
+
+        // 2. Set a null value
+        $e->name = null;
+
+        $this->handler()->serialize($e);
+
+        self::assertNull($e->_name);
     }
 
+
     private function handler(): SerializationHandler
     {
         return new SerializationHandler(