TRUNK-4565: Avoid a null pointer exception when validating Stop Orders for DrugOrder...

...s without a Drug.


It is valid to have a DrugOrder without a Drug, as long as there is a Concept.  The logic on lines 184-192 addresses that case.  However, line 179 was throwing NPE's when trying to stop a DrugOrder without a Drug.

See issue here https://issues.openmrs.org/browse/TRUNK-4565
I think this has changed the logic somehow, if the drug field was null before we need to ensure that is is still null but looks like with this change it isn't the case