If called without an argument the current (or the new DN if set) DN gets
returned.
If you provide an DN, this entry is moved to the new location specified
if a DN existed.
If the DN was not set, the DN gets initialized. Call {@link update()} to
actually create the new entry in the directory.
To fetch the current active DN after setting a new DN but before an
update(), you can use {@link currentDN()} to retrieve the DN that is
currently active.