aci_bd_subnet: Support parameter scope not set (#42675)

If parameter scope is not set (is None) the usual handling of lists does
not apply, and should be avoided or we get an exception.

This needs to be backported to earlier versions of Ansible.

lib/ansible/modules/network/aci/aci_bd_subnet.py

@@ -358,10 +358,11 @@ def main():
     route_profile = module.params['route_profile']
     route_profile_l3_out = module.params['route_profile_l3_out']
     scope = module.params['scope']
-    if 'private' in scope and 'public' in scope:
-        module.fail_json(msg="Parameter 'scope' cannot be both 'private' and 'public', got: %s" % scope)
-    else:
-        scope = ','.join(sorted(scope))
+    if scope is not None:
+        if 'private' in scope and 'public' in scope:
+            module.fail_json(msg="Parameter 'scope' cannot be both 'private' and 'public', got: %s" % scope)
+        else:
+            scope = ','.join(sorted(scope))
     state = module.params['state']
     subnet_control = module.params['subnet_control']
     if subnet_control: