fix: filter out ARP entries with empty MAC addresses from SNMP

- Added guard clause to return nil for empty binary MAC addresses
- Filter rejects entries with nil IP or MAC addresses
- Prevents 'can't be blank' validation errors on upsert
- Root cause: some SNMP devices return incomplete ARP entries

lib/towerops/snmp/arp_discovery.ex

@@ -46,7 +46,7 @@ defmodule Towerops.Snmp.ArpDiscovery do
           last_seen_at: DateTime.truncate(DateTime.utc_now(), :second)
         }
       end)
-      |> Enum.reject(&is_nil(&1.ip_address))
+      |> Enum.reject(&(is_nil(&1.ip_address) or is_nil(&1.mac_address)))
 
     {:ok, arp_entries}
   end
@@ -106,6 +106,11 @@ defmodule Towerops.Snmp.ArpDiscovery do
   end
 
   # Format MAC address from binary to colon-separated hex
+  defp format_mac_address(value) when is_binary(value) and byte_size(value) == 0 do
+    # Empty binary - SNMP returned blank MAC address
+    nil
+  end
+
   defp format_mac_address(value) when is_binary(value) and byte_size(value) == 6 do
     value
     |> :binary.bin_to_list()