There’s an if statement around the do while loop on that page, so that the do while loop also will not run if *count == 0.

The code snippets

    for (int i = 0; i < *count; ++i) {
        str[i] = 0;
    }

and

  if (*count > 0) {
    long idx = 0;
    do {
      str->__data[idx] = 0;
      idx += 1;
    } while (idx <= *count);
  }

Will produce the same changes to the string data given the same count.