[Testing] Fix nullable entitymanager in functional repository tests

[c0nst4ntin]

In Version 6.4 the a type was introduced to the $entityManager property in this example.
In the tearDown() method the entityManager is set to null ($this->entityManager = null;).

Docs (v5.x): https://symfony.com/doc/5.x/testing/database.html#functional-testing-of-a-doctrine-repository
Docs (v6.4): https://symfony.com/doc/6.4/testing/database.html#functional-testing-of-a-doctrine-repository

Currently:

private EntityManager $entityManager;

Change:

private ?EntityManager $entityManager;

[javiereguiluz]

Good catch Constantin! Thanks and congrats on your first Symfony Docs contribution 🎉