code: public static <T> Stream<T> ofSingleNullable(T nullable) {
          return null == nullable ? Stream.empty() : Stream.of(nullable);
      }
docs: Converts nulls into an empty stream, and non-null values into a stream with one element.
  @param nullable The nullable value to convert.
  @param <T> The type of the value.
  @return A stream of zero or one values.