Detchar cache shouldn't be returned as empty if there's nothing in /dev/shm/kafka

Right now if no files are in the llhoft /dev/shm/kafka location, an empty cache gets returned. But I think this should not get returned so that it can fall back to gwdatafind. https://git.ligo.org/emfollow/gwcelery/-/blob/main/gwcelery/tasks/detchar.py?ref_type=heads#L82