[Reference][Constraints] validate `null` (Expression constraint in 2.6)

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#11709)
| Applies to    | 2.6
| Fixed tickets | master part of #4191

Since Symfony 2.6, the Expression constraint doesn't skip validating `null` values.

Probably, it's a good idea to merge #4202 first and rebase this then to avoid merge conflicts.
Yes, I think we should just remove the example and make sure the message is very clear:

```rst
.. versionadded:: 2.6
    In Symfony 2.6, the Expression constraint *is* executed if the value is ``null``.
    Before 2.6, if the value was ``null``, the expression was never executed and
    the value was considered valid (unless you also had a constraint like `NotBlank`
    on the property).
```

This seems a bit wordy, but clear. It's a bit confusing because I don't immediately understand what "the expression was never executed" means. Does it mean it was always considered valid? Invalid?