Bug - AlertGroup - li elements never unmount for timedout alerts

Describe the problem
When Alerts with a timeout are rendered inside an AlertGroup, the wrapper li element for the Alerts never unmounts when the Alerts do. This causes empty li elements to remain in the DOM.

How do you reproduce the problem?
Go to the alert timeout example and add an alert in the example. Inspect the DOM and notice that when the Alert itself unmounts, the wrapper li element remains with nothing inside of it. Add another Alert and notice that the empty li still remains in addition to a new li with the new Alert rendered inside of it.

Expected behavior
The li elements would unmount the same as the Alerts themselves

Is this issue blocking you?
List the workaround if there is one.

Screenshots
If applicable, add screenshots to help explain the issue.

What is your environment?

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

What is your product and what release date are you targeting?

Any other information?

从 AlertGroup 组件开始，使用 alert timeout 示例重现该问题。在超时的 Alerts 消失时检查 DOM，并追踪它们的包装 li 元素是如何渲染和移除的。当超时的 Alerts 不再留下空的 li 元素时即表示完成，包括添加另一个 Alert 的情况。