HPCC-9029 Roxie can core if write to file when no workunit

A Roxie query that writes to a file can core while trying to publish the fine
info, if there is no workunit available (e.g. the query was received via a
socket)

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #4161 from richardkchapman/roxie-write-core

HPCC-9029 Roxie can core if write to file when no workunit

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>