docs: public static <T> Function<Object, /*@Nullable*/ T> returnNull() {
      // It is safe to cast a producer of Void to anything, because Void is always null.
      @SuppressWarnings("unchecked")
      Function<Object, /*@Nullable*/ T> function = (Function<Object, /*@Nullable*/ T>) RETURN_NULL;
      return function;
    }