fix(runtime-dom): handle null/undefined handler in withModifiers

[edison1105]

close #14361

Summary by CodeRabbit

• Bug Fixes
  • Improved stability of event handler modifiers by gracefully handling null or undefined values, preventing errors and ensuring consistent behavior in edge cases.

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[coderabbitai]

📝 Walkthrough

Walkthrough

This PR fixes a bug where undefined event handlers with modifiers in loops cause rendering errors. The solution adds an early guard in withModifiers to return falsy handlers unchanged, plus test cases verifying the graceful handling.

Changes

Cohort / File(s)	Summary
Event Handler Modifier Guard packages/runtime-dom/src/directives/vOn.ts	Added early return guard if (!fn) return fn to short-circuit withModifiers when handler is null/undefined, preventing cache initialization and function wrapping
Event Handler Modifier Tests packages/runtime-dom/__tests__/directives/vOn.spec.ts	Added test cases verifying withModifiers gracefully handles null (returns null) and undefined (returns undefined) handler inputs without throwing

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~5 minutes

Suggested labels

ready to merge, :hammer: p3-minor-bug

Suggested reviewers

• KazariEX

Poem

A guard in the flow, so simple and neat,
Falsy handlers now face defeat! 🐰
No more crashes in loops divine,
With undefined tamed, all tests align! ✨

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main change: adding a guard to handle null/undefined handlers in withModifiers, which directly addresses the PR's primary objective.
Linked Issues check	✅ Passed	The PR implements the required fix by adding an early guard in withModifiers to handle null/undefined handlers, directly addressing issue #14361 and matching the expected behavior from similar issue #9865.
Out of Scope Changes check	✅ Passed	All changes are directly related to fixing the null/undefined handler issue in withModifiers; the test case and runtime guard are both in scope.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing touches

• 📝 Generate docstrings

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[github-actions]

Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	103 kB (+15 B)	39.1 kB (+6 B)	35.2 kB (+14 B)
vue.global.prod.js	161 kB (+15 B)	59.1 kB (+6 B)	52.6 kB (+65 B)

Usages

Name	Size	Gzip	Brotli
createApp (CAPI only)	47 kB	18.4 kB	16.8 kB
createApp	55.1 kB	21.4 kB	19.6 kB
createSSRApp	59.4 kB	23.2 kB	21.1 kB
defineCustomElement	60.8 kB	23.1 kB	21.1 kB
overall	69.5 kB	26.7 kB	24.3 kB

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commit: c88267b