@Override
    public CodeBlock toProxy(CodeBlock expression) {
      return CodeBlock.of(
          "$L.isPresent() ? $L : null",
          expression,
          containedTypeSerializer.toProxy(CodeBlock.of("$L.get()", expression)));
    }
Creates a {@link Serializer} that supports {@link Optional} types.
value/src/main/java/com/google/auto/value/extension/serializable/serializer/impl/OptionalSerializerExtension.java