HPCC-10001 If classpath not set use default

Helpful on environments that have been upgraded from 3.x

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #4836 from richardkchapman/default-classpath

HPCC-10001 If classpath not set use default

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>