HPCC-8495 openWorkUnit() should return NULL for a NULL or empty wuid

IWorkUnitFactory::openWorkUnit builds an xpath to open the given
workunit in dali. If a blank wuid is provided, it opens the root
workunits tree as a workunit. Attempting to open null or empty
wuid should fail. By this fix, the openWorkUnit() returns NULL
for a NULL or empty wuid.

Signed-off-by: Kevin Wang <kevin.wang@lexisnexis.com>