Fix the API call in clean_docker_images role

roles/clean_docker_images/tasks/main.yml

@@ -12,7 +12,7 @@
           api_endpoint=netbox_api,
           token=netbox_token,
           validate_certs=False,
-          api_filter='containers=[] name=' + rps_images_prefix,
+          api_filter='containers=null name=' + rps_images_prefix,
         )
       ] | flatten
      }}