ExpressionLanguage: Variable values have to be passed to the parser

[xabbuh]

When an expression is parsed using the ExpressionLanguage::parse() method, you will have to pass the actual variable values (see symfony/symfony#12800). This means that you have to pass an empty array when the expression doesn't contain any variables. This is currently wrong at least in the examples shown on http://symfony.com/doc/2.5/components/expression_language/caching.html#using-parsed-and-serialized-expressions.