Section 4.3 talks of empty elements: http://docs.oasis-open.org/ubl/csprd01-UBL-2.2/UBL-2.2.html#S-EMPTY-ELEMENTS
But we don't say anything about empty attributes. I think the same arguments apply, and some attributes (such as the unit code for measurements) are mandatory. I think they should not be allowed to be empty.
Adding this would add a section 4.5 and new INDxx rules.