fix bug

src/workflow.py

@@ -33,6 +33,8 @@ class Workflow:
         self.keywords = keywords
         self.producer = producer
         self.publisher = publisher
+        if(processes_2_remove==""):
+            processes_2_remove = None
         self.processes_2_remove = processes_2_remove
         self.log = ""
         self.fill_log()