Fix missing variable with nxos_install_os tests (#60351)

Today we are getting an Ansible templating exception because this
variable isn't defined. But we also don't want testing to run, as we
don't have ability to run this test in DCI.

Signed-off-by: Paul Belanger <pabelanger@redhat.com>

test/integration/targets/nxos_install_os/defaults/main.yaml

@@ -1,2 +1,3 @@
 ---
-testcase: " {{ testcase }}"
+testcase: ""
+test_items: []