Distinguish the cases of "zero files in dataset", "number of files is unknown", and "dataset does not exist"

[ponyisi]

There seem to be multiple places in the user interface (e.g. the dashboard) where a dataset having zero files (e.g. when a Rucio dataset does not exist) is reported as "Unknown".

In the case of Rucio DIDs this is often because the dataset does not exist, which we should handle in some more obvious way than reporting an "Unknown" number of files. In some cases the Rucio dataset might exist but be empty, in which case we should properly report zero files.