For parameters where you aren't transferring ownership, just use raw pointers/references. If it's a required parameter make it a reference. If it's optional, use a pointer and check for null.
That's on the client code. You can't stop callers from doing something stupid (or cosmic rays from flipping bits). At that point crashing fast is preferable.
That's theoretic talk. Try doing it in practice and still get things done / be able to maintain the code / see the forest for the trees.
But I have a feeling that we are lacking a bit of context here. Some people seem to focus on web-application style of programming (understandably) where you have lots of trust issues. Whenever data is carried across trust boundaries it needs to be checked (this applies to integrity in general, of which null safety is just a small part).
(On the other hand, deserialization is not about validation of function arguments. Deserializers should assert integrity on the spot before calling into deeper nested functions).
