After the tabs component closes all tabs, the Add button disappears

[ziqiangai]

Reproduction link

https://stackblitz.com/edit/angular-m9djgw

Steps to reproduce

There is a problem with the tabs component. When opening and closing the tab page and the Add button, if all the open tab pages are closed, the Add button will not be found. This problem can be repeated in the official website document demo.

Tabs组件有一个问题，当开启关闭tab页和添加按钮时，如果把开启的tab页全部关掉，添加按钮也找不到了，这个问题在官网文档demo中即可复现。

What is expected?

关闭所有tabs选项卡应该还可以显示添加按钮

What is actually happening?

关闭所有tabs选项卡之后添加按钮就不见了

Environment	Info
ng-zorro-antd	12.0.2
Browser	Goole Chrome

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#7076

我尝试修复了这个问题，这是pr的地址。