Too narrow typehint on QueryConstraint::asQueryConstraint (?object)

[Brammm]

The QueryConstraint::asQueryConstraint method has an optional object as typehint. I assume this is because most ExpressionBuilder methods return some kind of Doctrine\ORM\Query\Expr class. However Expr::isNull and Expr::isNotNull return a string, not an object.

Should this typehint be made broader to simply mixed? I can make a PR if you'd like.