WorkflowTool.py: only convert transition variable when there exist transition.

product/ERP5/Tool/WorkflowTool.py

@@ -481,6 +481,7 @@ class WorkflowTool(BaseTool, OriginalWorkflowTool):
         variable.setInitialValue(vdef.default_value)
         variable.setDescription(vdef.description)
       # configure transition variable
+      if dc_workflow.transitions is not None:
         for tid in dc_workflow.transitions:
           origin_tdef = dc_workflow.transitions[tid]
           transition = workflow._getOb('transition_'+tid)