patches/DCWorkflow.py: get transition guard expression text instead of callable object in showAsXML.

deal with empty transition guard expression in conversion.

WorkflowTool.py: resolve the error when state has no premission_roles, matrix index should not be generated.

erp5_workflow: remove unecessesary tales in transition expression. tale can be pass as string type in this case.

WorkflowTool.py: in conversion, add state index conversion to fix premission roles matrix showing problem.