HPCC-12293 Roxie cores if indexFileName is null

Need to check for case where index is provided by an activity

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #6494 from richardkchapman/roxie-core-index-activity

HPCC-12293 Roxie cores if indexFileName is null

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>