Null Pointer Exception for some DrugOrders

To stop a DrugOrder, there is logic in the OrderService provided by OpenMRS to make sure you haven't changed the underlying drug. Unfortunately, the OpenMRS OrderService throws a Null Pointer if there is no drug on the drug order (even if there is a concept on the drug order).

It looks like I can remove lines 178-180 from the OrderServiceImpl, since the logic is duplicated at lines 184-192, with an appropriate null check (https://github.com/openmrs/openmrs-core/blob/1.10.x/api/src/main/java/org/openmrs/api/impl/OrderServiceImpl.java#L178).

It should be valid to have a DrugOrder that has a Concept but no Drug.