fix(cascader): fix custom empty slot not working in some cases

[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

自定义插槽在空数据的时候没有生效

Solution

How is the change tested?

Changelog

Component	Changelog(CN)	Changelog(EN)	Related issues
cascader	修复自定义空插槽部分情况下不生效问题	fix custom empty slot not working in some cases	Closes #3301

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

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