fix: handle missing vary header values #15947

Job	Run time
dependency-review	9s
Lint	38s
Fuzzing	3m 26s
Test TypeScript types	35s
Test with Node.js 20 compiled --without-intl	4m 22s
Test with Node.js 22 compiled --without-intl	4m 19s
Test with Node.js 23 compiled --without-intl	4m 19s
Test with SQLite enabled	23s
test (20, macos-latest) / Test with Node.js 20 on macos-latest	4m 9s
test (20, ubuntu-latest) / Test with Node.js 20 on ubuntu-latest	3m 45s
test (20, windows-latest) / Test with Node.js 20 on windows-latest	5m 27s
test (22, macos-latest) / Test with Node.js 22 on macos-latest	3m 18s
test (22, ubuntu-latest) / Test with Node.js 22 on ubuntu-latest	4m 11s
test (22, windows-latest) / Test with Node.js 22 on windows-latest	5m 50s
test (23, macos-latest) / Test with Node.js 23 on macos-latest	3m 19s
test (23, ubuntu-latest) / Test with Node.js 23 on ubuntu-latest	3m 28s
test (23, windows-latest) / Test with Node.js 23 on windows-latest	4m 52s
automerge	0s
	56m 30s

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