Where statically discernible, at present javac disallows assignment of null reference to a value instance. In the L-World prototype, this behavior needs
to change:

(1) It is an error to assign the null reference to a static or instance field that has been declared to be ACC_FLATTENABLE
(2) It is an error to update a field using withfield if the field has been declared to be ACC_FLATTENABLE
(3) If the actual type of the components of an array is a value class type, it is illegal to assign null to an element

At other places, null assignment to value instances is OK. (locals, parameters ... fields not tagged ACC_FLATTENABLE)