update 优化发布流程定义抛出异常

- StringUtils.join 默认会跳过 null，不会抛异常

ruoyi-modules/ruoyi-workflow/src/main/java/org/dromara/workflow/service/impl/FlwDefinitionServiceImpl.java

@@ -128,7 +128,7 @@ public class FlwDefinitionServiceImpl implements IFlwDefinitionService {
                 }
             }
             if (CollUtil.isNotEmpty(errorMsg)) {
-                throw new ServiceException("节点【{}】未配置办理人!", String.join(",", errorMsg));
+                throw new ServiceException("节点【{}】未配置办理人!", StringUtils.join(errorMsg, ","));
             }
         }
         return defService.publish(id);