feature symfony#4202 [Reference][Constraints] caution on null values in Expression constraint (xabbuh)

This PR was merged into the 2.4 branch.

Discussion
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[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 symfony#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.

Commits
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070489b caution on `null` values in Expression constraint

Author: weaverryan

reference/constraints/Expression.rst

@@ -217,6 +217,13 @@ more about the expression language syntax, see
                 // ...
             }
 
+    .. caution::
+
+        In Symfony 2.4 and Symfony 2.5, if the property (e.g. ``isTechnicalPost``)
+        were ``null``, the expression would never be called and the value
+        would be seen as valid. To ensure that the value is not ``null``,
+        use the :doc:`NotNull constraint </reference/constraints/NotNull>`.
+
 For more information about the expression and what variables are available
 to you, see the :ref:`expression <reference-constraint-expression-option>`
 option details below.