making strings from lists doesn't like 'None'

services/workflow/workflow/server.py

@@ -572,7 +572,8 @@ class WorkflowRequestRestService:
         req_list = self.request.info.lookup_stale_requests(int(keep))
 
         for r in req_list:
-            dir_list.append(r.results_dir)
+            if r.results_dir:
+                dir_list.append(r.results_dir)
 
         # return list as comma seperated string
         return Response(json_body={"stale_dirs": str(",".join(dir_list))})