Currently, when there are no edge ids, 0 is passed, e.g., in forEdges or forNeighbors. I think passing 0 if there are no edge ids is dangerous as it might lead to unexpected bugs if users have an edge attribute and just always read the first value because no edge ids are present in the graph. Unless there is a good reason to keep 0, I would suggest to change this to none so at least you notice that the value is not a valid edge id. This also affects #734 as it introduces new functionality also passing 0 as edge id.