docs: @Nullable
      private String normalizedDn(String dn) {
          if (isNullOrEmpty(dn)) {
              return dn;
          } else {
              try {
                  return new Dn(dn).getNormName();
              } catch (LdapInvalidDnException e) {
                  LOG.debug("Invalid DN", e);
                  return dn;
              }
          }
      }