validate null (Expression constraint in 2.6)

Since Symfony 2.6, the Expression constraint doesn't skip validating
`null` values.

Author: xabbuh

reference/constraints/Expression.rst

@@ -217,6 +217,12 @@ more about the expression language syntax, see
                 // ...
             }
 
+    .. versionadded:: 2.6
+        In Symfony 2.6, the Expression constraint *is* executed if the value
+        is ``null``. Before 2.6, if the value was ``null``, the expression
+        was never executed and the value was considered valid (unless you
+        also had a constraint like `NotBlank` on the property).
+
 For more information about the expression and what variables are available
 to you, see the :ref:`expression <reference-constraint-expression-option>`
 option details below.