[Bug] 基础表 设置表格分组展示时 导出excel报错

Version

1.19.7

Link to Minimal Reproduction

编辑器打不开 可以直接在官网的demo中复现：https://visactor.com/vtable/demo/export/table-export

Steps to Reproduce

表格初始化设置分组展示后 当表格中数据展示时 使用插件导出excel会报错
分组配置如下：

groupConfig:{
groupBy:'',
}

插件配置如下：
const tableExportPlugin = new VTablePlugins.TableExportPlugin({
exportExcelOptions: {
downloadFile: true,
fileName: 'fff-excel'
}
});

Current Behavior

当使用tableInstance.exportToExcel() 时 报错提示cannot merge already merged cells

Expected Behavior

正常导出 不报错

Environment

- OS:
- Browser:
- Framework:

Any additional comments?

No response

Start with the table export demo at https://visactor.com/vtable/demo/export/table-export and reproduce the error using groupConfig with groupBy set to an empty value and TableExportPlugin with exportToExcel(). Trace the grouped-table export path responsible for the "cannot merge already merged cells" error; done means grouped data exports to Excel without an exception.