ERP5Workflow: during workflow conversion only setVariableValue if default_value

setVariableValue sets value to None if parameter is empty string
this breaks testWorkflowAndDCWorkflow and change the expected behaviour
take care that if someone saves a variable without editing this field, variable_value
may be None again

product/ERP5Workflow/Tool/WorkflowTool.py

@@ -524,7 +524,7 @@ class WorkflowTool(BaseTool, OriginalWorkflowTool):
             interaction.setTriggerType(tdef.trigger_type)
             interaction.setDescription(tdef.description)
 
-        # create variables (portal_type = Variable)
+        # create variables (portal_type = Workflow Variable)
         for variable_id, variable_definition in dc_workflow.variables.items():
           variable = workflow.newContent(portal_type='Workflow Variable', temp_object=is_temporary)
           variable.setTitle(variable_definition.title)
@@ -547,7 +547,8 @@ class WorkflowTool(BaseTool, OriginalWorkflowTool):
               variable.setGuardExpression(tdef.info_guard.expr.text)
           variable.setForCatalog(variable_definition.for_catalog)
           variable.setStatusIncluded(variable_definition.for_status)
-          variable.setVariableValue(variable_definition.default_value)
+          if variable_definition.default_value:
+            variable.setVariableValue(variable_definition.default_value)
           variable.setDescription(variable_definition.description)
         # Configure transition variable:
         if getattr(dc_workflow, 'transitions', None) is not None: