Fix SegFault when Compute-Trajectories don't have any Auxiliary-Variables
requested to merge maximilian.hartz/met.3d:Fix_SegFault_Trajectories_without_auxiliary_variable into master
Fixes a SegFault when the Input Field "Trace auxiliary variables along trajectories" (new Dataset -> Trajectories -> Computation) is empty. This would result in a request for the variable "" (empty String) which does not exist.