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 +217,18 @@ more about the expression language syntax, see
                 // ...
             }
 
+    .. versionadded:: 2.6
+        Since Symfony 2.6, the Expression constraint doesn't skip ``null``
+        values.
+
+    Before Symfony 2.6, the expression would never be called if the property
+    (e.g. ``isTechnicalPost``) were ``null``. Now, if you want to allow ``null``
+    values, you'll have write your expression like this:
+
+    .. code-block:: text
+
+        this.getCategory() === null or this.getCategory() in ["php", "symfony"] or !this.isTechnicalPost()
+
 For more information about the expression and what variables are available
 to you, see the :ref:`expression <reference-constraint-expression-option>`
 option details below.