Null isn’t necessarily a reasonable default value for a pointer. Some users of pointers may expect a potential null; other users may expect a valid pointer and explode on a null.
I don’t want to suggest we shouldn’t do this, more that I can see arguments both ways. Do you have a specific use case in mind for this?
It might make more sense, for instance, to impl Default for a struct containing a pointer field, where you know the field can reasonably contain a null pointer.