rgw: rgw-orphan-list can continue with empty intermediate file(s)

rgw: rgw-orphan-list can continue with empty intermediate file(s)

rgw-orphan-list would exit with an error if either of the intermediate
files were empty. That's not necessarily indicative of an error,
though. If otherwise all the buckets have been removed then the
radosgw-admin intermediate file *should* be empty and the tool will
still find orphans. When an empty intermediate file is found, this
changes the output from error to a warning and will not exit.

Signed-off-by: J. Eric Ivancich <ivancich@redhat.com>