This worked for me in CP 5.8:
Ingest an empty text file (0KB in size) into a repository called Fileshare1.
Wait for ingestion to fully complete.
Access Administration > Settings > General > Fields to add Filesize to the list of fields (if it is not already there).
Restart CP engine.
Create a training category referencing repository Fileshare1 with
Filesize is equal to 0.
This returned the file that was 0KB in size.