dns: silence log for decommissioned devices

• When a device is decommissioned it ok for it to still have a mgmt IP address assigned but it will not be present in the list of devices. This currently generates a log that is unnecessary. Log it only if the IP has also a DNS name set.

Bug: T320721
Change-Id: I3e8e991b7c29eafd6f41b91d4e67400ed2dd75bb