code: public static <T> java.util.@Nullable Optional<T> toJavaUtil(
        @Nullable Optional<T> googleOptional) {
      return googleOptional == null ? null : googleOptional.toJavaUtil();
    }
docs: Returns the equivalent {@code java.util.Optional} value to the given {@code
  com.google.common.base.Optional}, or {@code null} if the argument is null.
  
  <p>If {@code googleOptional} is known to be non-null, use {@code googleOptional.toJavaUtil()}
  instead.
  
  <p>Unfortunately, the method reference {@code Optional::toJavaUtil} will not work, because it
  could refer to either the static or instance version of this method. Write out the lambda
  expression {@code o -> Optional.toJavaUtil(o)} instead.
  
  @since 21.0