Different validation rules on same entity in Symfony2

The admin will be able to edit the users of his website, but not the password. So I've created this EditUserFormType :

namespace CAPUserBundle\Form\Type;

use CAPShopAdminBundle\Repository\DiscountGridRepository;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolver;
use Symfony\Component\Form\Extension\Core\Type\TextType;
use Symfony\Component\Form\Extension\Core\Type\EmailType;
use Symfony\Component\Form\Extension\Core\Type\ChoiceType;
use Symfony\Bridge\Doctrine\Form\Type\EntityType;
use Symfony\Component\Form\Extension\Core\Type\SubmitType;

class EditUserType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder->add('email', EmailType::class);
        $builder->add('discountGrid', EntityType::class, array(
            'class' => 'CAPShopAdminBundle:DiscountGrid',
            'choice_label' => 'name',
            'placeholder' => '-- Aucune --',
            'required' => false,
            'query_builder' => function (DiscountGridRepository $r) {
                $qb = $r->createQueryBuilder('d');
                $qb->orderBy('d.name', 'ASC');
                return $qb;
            },
            'attr' => array("autocomplete" => "off"),
        ));
        $builder->add('isActive', ChoiceType::class, array(
            'choices' => array(
                'Actif' => '1',
                'Désactivé' => '0'
            ),
        ));
        $builder->add('name', TextType::class);
        $builder->add('firstname', TextType::class);
        $builder->add('company', TextType::class);
        $builder->add('address', TextType::class);
        $builder->add('postcode', TextType::class);
        $builder->add('city', TextType::class);
        $builder->add('phone', TextType::class);
        $builder->add('fax', TextType::class);

        $builder->add('save', SubmitType::class);
    }

    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults([
            'data_class' => 'CAPUserBundle\Entity\User',
        ]);
    }

    public function getName()
    {
        return 'edit_user_form';
    }
} 

But when i validate the form, there is an error with the plainPassword field from my User entity.

This value should not be blank.     edit_user   
Symfony\Component\Validator\ConstraintViolation

Object(Symfony\Component\Form\Form).data.plainPassword = 

Indeed, there is a NotBlank validation constraint on it in order to force the user to set the password when he is registering.

My question is : how can I bypass this constraint for editing my User ? The password field isn't present on the EditUserFormType, so the plainPassword is useless. But I'm stuck because I use the same User Entity where is set the NotBlank value.

For the moment, I force the plainPassord field with :

$user->setPlainPassword('Ap@ssw0rd');

But this is an awful way to bypass the constraint...