Symfony version(s) affected: 4.4.1

Description
Using the form_login authentification provider with a database user provider, got an error when login with a user with a null password (in the entity/database) :

Argument 1 passed to Symfony\Component\Security\Core\Encoder\NativePasswordEncoder::needsRehash() must be of the type string, null given, called in ...\vendor\symfony\security-core\Encoder\MigratingPasswordEncoder.php on line 51

This works in Symfony 4.1.

How to reproduce
Create an application with

• A User Entity
• A form and a controller for login
• Configure security

security:
    encoders:
        App\Entity\User:
            algorithm: 'bcrypt'
   providers:
        our_db_provider:
            entity:
                class: App\Entity\User 
   firewalls:
        main:
            anonymous: true
            form_login:
                login_path: login
                check_path: login
            provider: our_db_provider

• Add a user with an empty password
• Try to authenticate with this user

Possible Solution
Make parameter $encoded nullable in Symfony\Component\Security\Core\Encoder\NativePasswordEncoder::needsRehash()

Additional context
In my case I have null password because some users are authenticate with the form_login (password is in the User entity) others are authenticate with form_login_ldap (in this case the password is null in the User entity).