fixed: 创建会签数据时排除了无发起人userId的场景，跨级派单current无userId

src/Hotline/FlowEngine/Workflows/WorkflowDomainService.cs

@@ -371,7 +371,7 @@ namespace Hotline.FlowEngine.Workflows
             if (isStartCountersign)
             {
                 var exists = workflow.Countersigns.Any(d =>
-                    !d.IsCompleted() && d.StarterId == current.UserId);
+                    !d.IsCompleted() && !string.IsNullOrEmpty(current.UserId) && d.StarterId == current.UserId);
                 if (exists)
                     throw new UserFriendlyException("该用户在当前流程存在未结束会签");
                 await StartCountersignAsync(current, workflow, currentStep, dto, flowAssignInfo.FlowAssignType,