Allow heterodyne data file merging to safely ignore empty files
Following #101 (closed), there are cases where empty heterodyned data files exist (they are created by the pipeline so that inputs files exist that can be transferred in case of checkpointing during a run) and are included in a list of files for merging. This causes a problem in that the empty files are not valid HDF5 files and therefore cause an error when trying to be read during merging. This should be fixed by ignoring empty files during merging.