Fix: Path-source-receiver-mapping fail due to order limit

When the order of the paths is limited, the path-source-receiver-mapping failed. The cause of this was that the order limit could not drop all the path data since it could not drop the NaNs while the path did not exist. (When the path does not exist, no order data is present, ie NaNs, as a result, the path cannot be dropped using the current simple implementation.) Instead the path data is filled with NaNs. This however caused the path-source-receiver-mapping to fail as only the case where one non-NaN ID could exist. If the path was filled with NaNs, there were zero IDs for that path.

This MR fixes this issue by adding another case for zero IDs for the path. This is not the most elegant solution but it works.