TRUNK-4403 Blank drug order frequencies and dosing units should be converted to null during upgrade

(cherry picked from commit 2fefe9576c8a5fb60fa92e1ad36f9d6aeafe3333)

Conflicts:
	api/src/test/java/org/openmrs/util/databasechange/Database1_9_7UpgradeTest.java

(cherry picked from commit 6460c0283b1758501c0a8ce5d53567c52eeefaba)