Fix: visualizations where only high order paths exist
If no paths for a lower combined order of the max combined order exist, i.e. no paths of combined order one but paths of combined order two exist, the material for the higher order paths will not be created (correctly). As a result of this, when exporting the visualisation, the higher-order paths reference a non-existent material and the program segvaults.
To fix this, all missing, lower-combined-orders are added to the line hierarchy. With this, the material indices are created correctly. As a result, the visualization export does not crash with a segvault.