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

@@ -128,6 +128,18 @@ One way to accomplish this is with the Expression constraint:
             // ...
         }
 
+.. versionadded:: 2.6
+    Since Symfony 2.6, the Expression constraint doesn't skip ``null`` values.
+
+Before Symfony 2.6, the Expression constraint didn't validate ``null`` values.
+To preserve this behavior, you'll have to allow ``null`` values explicitly.
+For example, to allow blog posts without a category, your expression will
+look like this:
+
+.. code-block:: text
+
+    this.getCategory() === null or this.getCategory() in ["php", "symfony"] or !this.isTechnicalPost()
+
 The :ref:`expression <reference-constraint-expression-option>` option is the
 expression that must return true in order for validation to pass. To learn
 more about the expression language syntax, see