fix(cascader): fix empty state in Cascader Virtual list

[oljc]

Types of changes

• New feature
• Bug fix
• Enhancement
• Component style change
• Typescript definition change
• Documentation change
• Coding style change
• Refactoring
• Test cases
• Continuous integration
• Breaking change
• Others

Background and context

开启虚拟列表时空数据不显示空状态 #2684

Solution

📌 为保持不影响以前的dom层级结构添加了外层。实际不加外层（或直接用div包裹）也许会更好看。

加（与以前保持一致）：

未添加（居中）：

How is the change tested?

Changelog

Component	Changelog(CN)	Changelog(EN)	Related issues
cascader	修复开启虚拟列表时空状态不显示	fix empty state in Cascader Virtual list	#2684

Checklist:

• Test suite passes (npm run test)
• Provide changelog for relevant changes (e.g. bug fixes and new features) if applicable.
• Changes are submitted to the appropriate branch (e.g. features should be submitted to feature branch and others
  should be submitted to main branch)

Other information