ERP5Workflow: set state_permission_roles to an empty dict instead of None

during workflow conversion, when original workflow permission_roles is
None

product/ERP5Workflow/Tool/WorkflowTool.py

@@ -370,10 +370,10 @@ class WorkflowTool(BaseTool, OriginalWorkflowTool):
             state.setTitle(sdef.title)
             state.setReference(sdef.id)
             state.setDescription(sdef.description)
-            permission_roles = sdef.permission_roles
+            permission_roles = sdef.permission_roles or {}
             state.setStatePermissionRoles(permission_roles)
-            if sdef.permission_roles is not None:
-              state.setCellRange(sorted(sdef.permission_roles.keys()),
+            if permission_roles:
+              state.setCellRange(sorted(permission_roles.keys()),
                     sorted(workflow.getManagedRoleList()),
                     base_id='cell')