Revision field should not be required (NotNull) on cps-ri YangResourceEntity

Right now that column has "NotNull" annotation cps/YangResourceEntity.java at master · onap/cps (github.com). We have some YANGs which doesn't contain "revision" field and because of that we can't create CM handler. Based on information from[ YANG 1.1 RFC|http://example.com|http://example.com]/] revision statement is not mandatory (A module SHOULD have at least one "revision" statement) CPS shouldn't require that field.

As discussed during CPS weekly meeting 23.9 that field should be stored as an empty string if it's not included in a YANG