Empty files should have a locality DISK

Currently, storm-tape returns a locality of NONE for empty files, as allowed by the specification. This behaviour, however, causes problems to some clients, that prefer to get locality DISK in such cases (see for example https://ggus.eu/index.php?mode=ticket_info&ticket_id=168538#update#26 and following comments).

I'm not convinced that the change is good for everyone (otherwise why allow NONE in the specification?), but let's try.