Modifier un utilisateur sans changer le mot de passe

Bonjour,

Voila je rencontre un petit problème avec mon code.

J'ai fais un système utilisateur à l'aide de symfony 4.
Je n'utilise pas FOSUserBundle, mais l'ai fait a la main avec la doc.

Sur la page édition de mon utilisateur, je voudrais que si je laisse le champs mot de passe vide, cela me laisse le mot de passe actuelle et ne fasse que la modification sur les autres champs.

Voici mon code de mon controller:

public function edit(Request $request, User $user, UserPasswordEncoderInterface $passwordEncoder): Response
    {
        $form = $this->createForm(EditUserFormType::class, $user);
        $form->handleRequest($request);

        if ($form->isSubmitted() && $form->isValid()) {
            // encode the plain password
            $user->setPassword(
                $passwordEncoder->encodePassword(
                    $user,
                    $form->get('plainPassword')->getData()
                )
            );

            $entityManager = $this->getDoctrine()->getManager();
            $entityManager->persist($user);
            $entityManager->flush();

            // do anything else you need here, like send an email

            return $this->redirectToRoute('index_user_back');
        }

        return $this->render('back/users/edit.html.twig', [
            'user' => $user,
            'registrationForm' => $form->createView()
        ]);
    }

Voici le code de mon form:

<?php

namespace App\Form\Back;

use App\Entity\User;
use Symfony\Component\Security\Core\User\UserInterface;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\Extension\Core\Type\PasswordType;
use Symfony\Component\Form\Extension\Core\Type\CheckboxType;
use Symfony\Component\Form\Extension\Core\Type\ChoiceType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolver;
use Symfony\Component\Validator\Constraints\NotBlank;
use Symfony\Component\Validator\Constraints\Length;
use Symfony\Component\Validator\Constraints\IsTrue;
use Symfony\Component\Form\CallbackTransformer;

class EditUserFormType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('email')
            ->add('name')
            ->add('firstname')
            ->add('plainPassword', PasswordType::class, [
                // instead of being set onto the object directly,
                // this is read and encoded in the controller
                'mapped' => false,
                'constraints' => [
                    new NotBlank([
                        'message' => 'Please enter a password',
                    ]),
                    new Length([
                        'min' => 6,
                        'minMessage' => 'Your password should be at least {{ limit }} characters',
                        // max length allowed by Symfony for security reasons
                        'max' => 4096,
                    ]),
                ],
            ])
            ->add('roles', ChoiceType::class, [
                'choices' => [
                    'Utilisateur' => 'ROLE_USER',
                    'Administrateur' => 'ROLE_ADMIN',
                ],
                'label' => 'Role :'
            ])
        ;

        //roles field data transformer
        $builder->get('roles')
            ->addModelTransformer(new CallbackTransformer(
                function ($rolesArray) {
                    // transform the array to a string
                    return count($rolesArray)? $rolesArray[0]: null;
                },
                function ($rolesString) {
                    // transform the string back to an array
                    return [$rolesString];
                }
            ));
    }

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

Merci d'avance.