all: Use NULL instead of "" when calling mp_raise exception helpers.

This is the established way of doing it and reduces code size by a little
bit.

ports/unix/modjni.c

@@ -266,7 +266,7 @@ STATIC mp_obj_t jobject_subscr(mp_obj_t self_in, mp_obj_t index, mp_obj_t value)
                 return mp_const_none;
             }
         }
-        mp_raise_NotImplementedError("");
+        mp_raise_NotImplementedError(NULL);
     }
 
     if (!JJ(IsInstanceOf, self->obj, List_class)) {