[Reference][Constraints] caution on `null` values in Expression constraint

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.4, 2.5
| Fixed tickets | part of #4191

This adds a warning to the reference of the Expression constraint explaining that `null` values won't be validated in Symfony 2.4 and Symfony 2.5.
it is about ``null`` in the field on which you apply the expression. this can only happen on property level. A class-level constraint can not be triggered with a ``null`` value (unless you do wonkey struff calling the validator manually with the class constraints) as ``null`` cannot be an instance of your class.