SFDC picklist fields should be able to be changed back to null value in GS UI

When a non-required SFDC picklist field is edited within the GS UI, you are not able to change the field back to null (in SFDC you are able to change a picklist field back to null once a value has been selected and saved). SFDC picklists should work the same between SFDC and the GS UI (this is especially useful for Attributes pages and Success Plans).