Dataset overview with empty dataset_names

Bugged dataset overview response, caused by ckanext/odsh/collection/helpers.py when dataset_names in get_datasets_from_solr is None or when datasets_in_collection in gather_collection_info function is None.

Example: https://opendata.schleswig-holstein.de/dataset/rba-statistik-arbeitsstand-jahr (not visible in active instance, because it has been manually fixed)