HPCC-10319 - Fix attr problems if a subfile is empty super

If the a subfile of a super was an empty superfile, the
file descriptor failed to set the common attributes, e.g.
@blockCompression. The net result was that the engines would
incorrectly think the super was uncompressed and hit
deserialization problems reading the rest of the files.

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>