presto-main/src/main/java/io/prestosql/sql/planner/ExpressionInterpreter.java/Visitor/visitIsNullPredicate
class Visitor:        @Override
        protected Object visitIsNullPredicate(IsNullPredicate node, Object context)
        {
            Object value = process(node.getValue(), context);

            if (value instanceof Expression) {
                return new IsNullPredicate(toExpression(value, type(node.getValue())));
            }

            return value == null;
        }