fix(avatar-group): fix partial avatars not displaying in the avatar group popover

[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

当省略的头像数量大于最大显示数时，z-index会成为负数导致无法正常显示。

Solution

How is the change tested?

Changelog

Component	Changelog(CN)	Changelog(EN)	Related issues
avatar-group	修复头像组气泡中部分头像不显示问题	fix partial avatars not displaying in the avatar group popover	#2745

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