Does any language have this feature?

Many languages that have references (pointers, whatever) have a distinguished null value that refers to "nowhere": None, a null pointer, undef etc. Dereferencing this value produces some kind of runtime error (a segmentation fault or an exception, for instance) so you have check every possible use of any reference that can ever be null. (In some cases it's acceptable to take the exception or even the crash, but often it not)

( What I'd like is a distinction between optional and mandatory references )