Univention Corporate Server 4.4 erratum 589 (bugfix)

This update addresses the following issue:
* Internal improvement: Remove excessive log trace information from UDM
  modules method `__getitem__()`.
* The Simple UDM API did not return an advanced list of DNs if the attribute
  was not set at all. It merely returned an empty list, which could lead to
  errors. This has been fixed.