A user ran into some confusion about the semantics of the is_null field of *Val classes in the UDF interface. The intended semantics are that if is_null is true, then no other fields in the class are valid. E.g. StringVal::len and StringVal::ptr.
However, this isn't explicitly documented anywhere and a user assumed that is_null == true implies that the other fields should be initialised to len == 0 and ptr == NULL