modffi: 's' (string) return type: handle NULL properly (return None).

unix/modffi.c

@@ -140,6 +140,9 @@ STATIC mp_obj_t return_ffi_value(ffi_arg val, char type)
     switch (type) {
         case 's': {
             const char *s = (const char *)val;
+            if (!s) {
+                return mp_const_none;
+            }
             return mp_obj_new_str(s, strlen(s), false);
         }
         case 'v':