Fixes a Fatal Error when using a subexpression in parenthesis

When some dql contains a subselect with expression in parenthesis, the
expression of the $simpleSelectExpression parameter given to
walkSimpleSelectExpression is an instance of AST\ParenthesisExpression.
Before this commit, this case defaulted to
$this->walkEntityIdentificationVariable($expr) where $expr is supposed
to be a string. A fatal error was then yielded.