validate `null` (Expression constraint in 2.6)

Since Symfony 2.6, the Expression constraint doesn't skip validating
`null` values.
feature #4204 [Reference][Constraints] validate `null` (Expression constraint in 2.6) (xabbuh)

This PR was merged into the master branch.

Discussion
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[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.

Commits
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fb18056 validate `null` (Expression constraint in 2.6)