fix(compiler): avoid empty if() guard for .exact with all sys modifiers

[Bojun-Vvibe]

Closes #12319

Repo

vuejs/vue

Issue

#12319

Root cause

In src/compiler/codegen/events.ts, the .exact modifier branch builds a guard condition by filtering out the system modifiers already present on the handler. When all four system modifiers (ctrl, shift, alt, meta) are listed alongside .exact, the filter yields an empty array and join('||') returns '', which is then wrapped by genGuard to produce the syntactically invalid if()return null;.

Fix

Compute the joined condition first and only emit genGuard(condition) when the condition is a non-empty string. With every system modifier already required by other modifier guards, the .exact guard becomes redundant and can be safely omitted.

Regression test

test/unit/modules/compiler/codegen.spec.ts — added an assertCodegen case for <button @keydown.ctrl.shift.alt.meta.exact="onClick"> asserting the generated render code contains the four individual if(!$event.XKey)return null; guards and no empty if() clause.

Risk

trivial

Verification

skipped: project devDependencies (vitest) are not installed in this fresh clone, and npx vitest cannot resolve vitest/config. Fix verified by inspection: the new branch only suppresses genGuard when its condition string is empty, leaving all existing behavior untouched, and the new codegen assertion mirrors the deterministic output expected from the surrounding generator.