26960bdd34
* On e.g. Cisco N9K, ipv6 objects can be returned undef which crashes the ip alias discovery with `Can't use an undefined value as a HASH reference at .../Discover/Properties.pm line 346.`
* Discovery continues but leaves device_ip empty, which in turn makes device->neighbor non-functional
* Fixed by adding various `$snmp->x || {}` safeguards