fix: Handle nil vlan_id in MAC address upsert queries

Fixes ArgumentError when upserting MAC addresses with nil vlan_id.
Ecto doesn't allow direct comparison with nil in queries - must use
is_nil/1 instead.

Changed upsert_mac_address to build dynamic query that uses is_nil(m.vlan_id)
when vlan_id is nil, or direct comparison when it has a value.

Error was:
  Comparison with nil is forbidden as it is unsafe.
  Instead write a query with is_nil/1

lib/towerops/snmp.ex

@@ -1563,11 +1563,22 @@ defmodule Towerops.Snmp do
   Creates or updates a MAC address entry.
   """
   def upsert_mac_address(attrs) do
-    case Repo.get_by(MacAddress,
-           device_id: attrs.device_id,
-           mac_address: attrs.mac_address,
-           vlan_id: attrs[:vlan_id]
-         ) do
+    import Ecto.Query
+
+    # Build query to find existing MAC address entry
+    query =
+      from m in MacAddress,
+        where: m.device_id == ^attrs.device_id,
+        where: m.mac_address == ^attrs.mac_address
+
+    # Handle vlan_id comparison (nil requires is_nil/1)
+    query =
+      case attrs[:vlan_id] do
+        nil -> where(query, [m], is_nil(m.vlan_id))
+        vlan_id -> where(query, [m], m.vlan_id == ^vlan_id)
+      end
+
+    case Repo.one(query) do
       nil ->
         %MacAddress{}
         |> MacAddress.changeset(attrs)