[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.
Like with #4202, I'm not totally clear on what the behavior is. I assume that in Symfony 2.6, my expression is *always* called, even if the underlying value is null. I think this would be the expected behavior, so I almost think we don't need the `versionadded` (we *do* have the warning on the 2.4 and 2.5 docs for its odd behavior).

But I *do* like mentioning (almost as a reminder) that your expression *will* be called, even if the value is null. And so, if you want to allow null values, you'll need something like this (then the example).

But also similar to #4202, does this apply when the constraint is applied to the entire object? Or does this only apply to the sidebar below?