test: GraphLive.Show error branches for missing sensor/storage/interface

Adds 4 tests that drive the nil → nil fallbacks in
load_single_sensor_chart_data, load_storage_chart_data,
load_interface_errors_chart_data, and load_interface_traffic_chart_data
by navigating to graph routes with non-existent IDs.

test/towerops_web/live/graph_live/show_events_test.exs

@@ -511,6 +511,54 @@ defmodule ToweropsWeb.GraphLive.ShowEventsTest do
       assert {:error, {:live_redirect, %{to: "/devices"}}} =
                live(conn, ~p"/devices/#{fake_id}/graph/processors")
     end
+
+    test "renders gracefully when sensor_id query param does not exist", %{
+      conn: conn,
+      device: device
+    } do
+      fake_sensor_id = Ecto.UUID.generate()
+
+      {:ok, _view, html} =
+        live(conn, ~p"/devices/#{device.id}/graph/temperature?sensor_id=#{fake_sensor_id}")
+
+      assert is_binary(html)
+    end
+
+    test "renders gracefully when storage_id query param does not exist", %{
+      conn: conn,
+      device: device
+    } do
+      fake_storage_id = Ecto.UUID.generate()
+
+      {:ok, _view, html} =
+        live(conn, ~p"/devices/#{device.id}/graph/storage_volume?storage_id=#{fake_storage_id}")
+
+      assert is_binary(html)
+    end
+
+    test "renders interface_errors with non-existent interface id", %{
+      conn: conn,
+      device: device
+    } do
+      fake_interface_id = Ecto.UUID.generate()
+
+      {:ok, _view, html} =
+        live(conn, ~p"/devices/#{device.id}/graph/interface_errors?interface_id=#{fake_interface_id}")
+
+      assert is_binary(html)
+    end
+
+    test "renders traffic with non-existent interface id", %{
+      conn: conn,
+      device: device
+    } do
+      fake_interface_id = Ecto.UUID.generate()
+
+      {:ok, _view, html} =
+        live(conn, ~p"/devices/#{device.id}/graph/traffic?interface_id=#{fake_interface_id}")
+
+      assert is_binary(html)
+    end
   end
 
   describe "interface_errors graph data shapes" do