[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.
You're right. Though I'm not sure if it does makes sense at all to explain the behaviour with the current example. What do you think?