HPCC-12334 Fix core if -main is used with no filename

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #6482 from ghalliday/issue12334

HPCC-12334 Fix core if -main is used with no filename

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>