HPCC-12738 Codegen should not assume Roxie output is always XML

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #6753 from richardkchapman/roxie-output-may-not-be-xml

HPCC-12738 Codegen should not assume Roxie output is always XML

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>