NullPointerException at PatientSetService#getPatientsHavingDrugOrder(java.util.Collection, java.util.Collection, GroupMethod, Date, Date)

If we call method PatientSetService#getPatientsHavingDrugOrder(java.util.Collection, java.util.Collection, GroupMethod, Date, Date) with following parameters, it will throw NullPointerException.

Parameters:
patientIds = null
takingIds = a collection list with number of element larger than zero
groupMethod = GroupMethod.NONE
fromDate = null
toDate = null

PR: https://github.com/openmrs/openmrs-core/pull/1790