static Copier notAcceptingNull(Function<String, String> copy) {
      return new Copier(copy, false);
    }
Specifies how to copy a parameter value into the target type. This might be the identity, or it
might be something like {@code ImmutableList.of(...)} or {@code Optional.ofNullable(...)}.
value/src/main/java/com/google/auto/value/processor/BuilderSpec.java