fix network map

lib/towerops/snmp.ex

@@ -1721,7 +1721,12 @@ defmodule Towerops.Snmp do
   defp build_node_lookup(devices, nodes) do
     # Create maps for matching by different attributes
     by_id = Map.new(nodes, &{&1.id, &1})
-    by_name = Map.new(nodes, &{String.downcase(&1.label), &1.id})
+
+    by_name =
+      nodes
+      |> Enum.filter(&(&1.label != nil))
+      |> Map.new(&{String.downcase(&1.label), &1.id})
+
     by_ip = Map.new(nodes, fn node -> {node.ip_address, node.id} end)
 
     # Build MAC address lookup from SNMP devices