fix: don't tear down IP assignments when guest.net is empty but tools report running

[dirtycache]

Summary

• BIG-IP VE hosts on old TMOS builds (11.5.0, 12.1.4.1) report VMware Tools as guestToolsRunning but never populate per-NIC guest.net data. Every sync cycle, this looked like "all IPs removed from all interfaces" and netbox-sync deleted the (correct, live) IP-to-interface assignments.
• Extends the existing skip_ip_handling guard (previously only triggered when tools aren't running at all) to also skip IP add/remove for a VM when guest.net comes back completely empty despite tools reporting running.
• A genuine per-NIC IP removal is unaffected: that case still reports the NIC in guest.net, just with no IP bound to it, so skip_ip_handling stays False and the removal still happens.
• Known gap: a VM that legitimately drops to zero reporting NICs (all interfaces at once) would also be protected by this guard and not reflected in NetBox. Considered acceptable — far less common than a single NIC losing its IP.

Test plan

• Confirm bigip-ve-001/bigip-ve-002 (the two affected hosts) stop losing their mgmt IP-to-interface assignment across sync cycles once deployed
• Confirm a genuine IP removal on a normal VM (e.g. unassign an IP from a vNIC with working tools) still gets reflected/removed in NetBox