fix(runtime-vapor): guard default slot for createPlainElement

[zhiyuanzmj]

Fix VaporRouterLink: https://github.com/vuejs/router/pull/2509/changes#diff-5bd820c9af09743190b2fce76977fc80da74307c554a4403f55a8c30a599214bR78

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commit: 04c92d6

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Size Report

Bundles

File	Size	Gzip	Brotli
compiler-dom.global.prod.js	86.4 kB	30.2 kB	26.6 kB
runtime-dom.global.prod.js	109 kB	41.4 kB	37.2 kB
vue.global.prod.js	169 kB	61.2 kB	54.5 kB

Usages

Name	Size	Gzip	Brotli
createApp (CAPI only)	49.5 kB	19.4 kB	17.7 kB
createApp	58.5 kB	22.7 kB	20.7 kB
createApp + vaporInteropPlugin	81.5 kB	30.3 kB	27.4 kB
createVaporApp	28.7 kB	11.1 kB	10.2 kB
createSSRApp	62.8 kB	24.4 kB	22.2 kB
createVaporSSRApp	32.1 kB	12.4 kB	11.4 kB
defineCustomElement	64.5 kB	24.6 kB	22.3 kB
defineVaporCustomElement	39 kB	14.4 kB	13.2 kB
overall	73.3 kB	28 kB	25.4 kB

[Copilot]

Pull request overview

This PR fixes a crash in createPlainElement when a default slot is not provided, which was causing issues in VaporRouterLink. The fix adds a null check before invoking the slot function.

Changes:

• Modified createPlainElement to guard against undefined default slots by checking the slot exists before calling it
• Updated existing test to properly pass slots through createPlainElement
• Added new test to verify that empty slots don't cause crashes

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
packages/runtime-vapor/src/component.ts	Added null check before invoking default slot function to prevent crashes when slot is undefined
packages/runtime-vapor/tests/customElement.spec.ts	Updated existing test to pass slots parameter and added new test to verify empty slots handling

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