updated default available machines value

gcc_exec/gcc_workflow.py

@@ -47,7 +47,7 @@ def __init__(self, gcc_user_obj: GccUser, workflow_name: str) -> None:
         }
 
     def plan(
-        self, available_machines: list = None, xml_specification: str = None
+        self, available_machines: list = [], xml_specification: str = None
     ) -> list:
         """This method creates an execution plan based on a workflow specification."""
         if xml_specification is None: