[9.x] Document that model encrypter can also be null

[axlon]

When using a custom encrypter for tests, you need to manually unset the encrypter after use (otherwise it might affect other tests). Since static properties cannot be unset with unset() the encrypter needs to be set to null (which is fine because models use ?? to use either a custom encrypter or the default).

This PR changes the documented type of the custom encrypter so PHPStan allows this:

protected function tearDown(): void
{
    MyModel::encryptUsing(null);
    parent::tearDown();
}