fix(runtime core) : improve slot type definitions

[Procrustes5]

closes #11671

Description

This PR enhances the Slot type definition to allow for more flexible return types.

Modified Slot type definition

Before: (...args: any[]) => VNode[]
After: (...args: any[]) => VNode[] | VNode | null | undefined

This change allows slot functions to return a single VNode, null, or undefined, better reflecting common patterns in actual Vue components.

Updated the normalizeSlot function to be compatible with the new Slot type.
Added type tests to ensure the new Slot type works as intended.

[github-actions]

Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	99.1 kB	37.5 kB	33.8 kB
vue.global.prod.js	157 kB	57.3 kB	51 kB

Usages

Name	Size	Gzip	Brotli
createApp	54.5 kB	21.1 kB	19.3 kB
createSSRApp	58.4 kB	22.8 kB	20.8 kB
defineCustomElement	59.1 kB	22.6 kB	20.6 kB
overall	68.1 kB	26.2 kB	23.8 kB

[pkg-pr-new]

commit: 91ec974

@vue/compiler-core

pnpm add https://pkg.pr.new/@vue/compiler-core@11725

@vue/compiler-dom

pnpm add https://pkg.pr.new/@vue/compiler-dom@11725

@vue/compiler-sfc

pnpm add https://pkg.pr.new/@vue/compiler-sfc@11725

@vue/compiler-ssr

pnpm add https://pkg.pr.new/@vue/compiler-ssr@11725

@vue/reactivity

pnpm add https://pkg.pr.new/@vue/reactivity@11725

@vue/runtime-core

pnpm add https://pkg.pr.new/@vue/runtime-core@11725

@vue/runtime-dom

pnpm add https://pkg.pr.new/@vue/runtime-dom@11725

@vue/server-renderer

pnpm add https://pkg.pr.new/@vue/server-renderer@11725

@vue/shared

pnpm add https://pkg.pr.new/@vue/shared@11725

vue

pnpm add https://pkg.pr.new/vue@11725

@vue/compat

pnpm add https://pkg.pr.new/@vue/compat@11725

Open in Stackblitz

[yyx990803]

Is this still needed as #11671 is closed?

[Procrustes5]

@yyx990803
Oh I've not realized the issue is closed.
Although the issue has been closed, I still think that the slot type can be better when it has a bit looser type. Could you advise me on what to do with this PR?