Guest checkout is missing email validation

With current implementation it is possible to checkout with empty or invalid email address as guest.

We need scenarios for missing or invalid email address too in https://github.com/Sylius/Sylius/pull/1816.
@stloyd Not sure, but I think emails are longer. `EmailValidator` relies on `filter_var($value, FILTER_VALIDATE_EMAIL)`, not sure what is max len for this filter, But I think it is more then 254. We have same on other places, if there is issue, we should open new one and fix them all.