Solving the oxymoron of nibbāna?
When the Heart Sutra says "There is no suffering, no cause of suffering, no end to suffering, no path to follow" it isn't saying that none of these things exist. What it means is that none of these things exist in terms of 'Svabhava' which means something like 'intrinsic existence' or 'self nature' or 'essence'. In terms of 'Svabhava' none of these things exist, but in terms of the experience of these things and in terms of their actual features and characteristics, they do exist.

And I think that the quote by Ryokan means that opposites are concepts which are relative and exist as abstract concepts which are imputed onto experience. It is absolutely true that the abstracted concept of Nibbana and the abstracted concept of conditioned existence relatively depend on each other as part of a dualistic pair, but that doesn't mean that the experiences and properties to which these concepts refer are themselves interdependent.