[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.
When I read this, the behavior isn't totally clear. Does my expression just never get called? Which value would be null for this behavior to happen? The entire `BlogPost`? Or does this only apply only when I'm using this constraint on a specific field (like in the sidebar below)? I think that's the case, and if it is, we should move it there. Then, let's be even more direct, something like:

>
In Symfony 2.4 and Symfony 2.5, if the property (e.g. `isTechnicalPost`) were null, then
the expression would never be called and the value would be seen as valid. To make
sure that the value is not null, use the :doc:`NotNull constraint </reference/constraints/NotNull>`.