Expression constraint: document the behaviour for null values

[xabbuh]

Until Symfony 2.5, the Expression constraint didn't validate null values. This should be made clear in the documentation for 2.4 and 2.5. However, this has been changed for the upcoming 2.6 release (see symfony/symfony#11709) and should be documented accordingly.

[wouterj]

You mean "until 2.5"? In that case, we should add a simple caution directive to the 2.4 - 2.5 branches explaining the problem in reference/constraints/expression.rst and add a versionadded in master.

[xabbuh]

Yeah, you're right (updated the description). By the way, 2.3 is not affected since the ExpressionLanguage was introduced in 2.4.

[wouterj]

Updated my comment too :)