http://docs.oasis-open.org/ubl/os-UBL-2.1/UBL-2.1.html#S-EMPTY-ELEMENTS
  "UBL takes the position that empty elements do not provide the level
   of assurance necessary for business information exchanges and
   therefore must not be used."

... and have reduced the requirement to the following rules:

   [IND5] UBL conformant instance documents MUST NOT contain an
          element devoid of content or containing null values,
          except in the case of extension, where the UBL
          ExtensionContent element is used.

   [IND6] The absence of a construct or data in a UBL instance
          document MUST NOT carry meaning.