code: public static <T> Collector<T, AtomicReference<T>, T> uniqueNullable() {
          return Collector.of(
                  AtomicReference::new,
                  CollectorUtils::uniqueAccumulate,
                  CollectorUtils::uniqueCombine,
                  AtomicReference::get
          );
      }
docs: A collector that returns the single member of a stream (or null if not present), or throws a
  {@link com.codepoetics.protonpack.collectors.NonUniqueValueException} if more
  than one item is found.
  @param <T> The type of the items in the stream.
  @return The collector.