Modify the DrugOrderValidator To Handle Free Text Orders Correctly.

The drug order validator enforces the requirement to have a drug set for all orders. This is not a desirable behavior for free text drug orders. 

Requirement

Modify the validator to properly take into account the free text orders by allowing the drug to be null while only enforcing the requirement to have the concept set.

Definition of Done

The drug order validator should not reject a free text order with drug set to null as a reason.

Associated talk thread -> https://talk.openmrs.org/t/error-saving-a-drug-order-with-non-coded-drug/14206