Symfony - How to deserialize with default values?

I'm starting to learn Symfony 4.

I want to deserialize datas from json data. (I'm using JMSSerializer)

This is my context : I have a Customer entity in src/App/Entity

class Customer {
    /**
     * 
     * @ORM\Id
     * @ORM\Column(type="integer")
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    private $id;
    /**
     * @ORM\Column(type="string", length=250)
     * @Assert\NotBlank()     
     */
    private $name;
     /**
     * @ORM\Column(type="string", length=250)
     * @Assert\NotNull
     */
    private $comment;

    public function __construct() {
    $this->comment = "";
    }
}

I have a CustomerController controller in src/App/Controller

class CustomerController extends Controller
{
    /**
     * @Route("/customers", name="customer_create")
     * @Method({"POST"})
     */
    public function createAction(Request $request)
    {
        $data = $request->getContent();
        //Il faudrait valider les données avant de les mettre en base de données
        $customer = $this->get('jms_serializer')->deserialize($data, 'App\Entity\Customer', 'json');        

        $em = $this->getDoctrine()->getManager();
        $em->persist($customer);
        $em->flush();

        return new Response('', Response::HTTP_CREATED);
    }
}

I post a request for creating a customer with the following JSON data:

{"name":"Customer Lambda"}

but I have the following error

NotNullConstraintViolationException

An exception occurred while executing 'INSERT INTO customers (id, name, comment) VALUES (?, ?, ?, ?)' with params [Resource id #99, "Customer Lambda", null]:

SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'comment' cannot be null

Is it possible to set a default value (empty string) for the comment field after deserialize? I was thinking that setting it in the Customer constructor will fix the issue but it is not the case.

1º Why are you trying to set empty string if you have defined comment as not nullable.

2º

This:

 /**
 * @ORM\Column(type="string", length=250)
 * @Assert\NotNull
 */
private $comment = "";

public function __construct() {
}

Instead this:

 /**
 * @ORM\Column(type="string", length=250)
 * @Assert\NotNull
 */
private $comment;

public function __construct() {
$this->comment = "";
}