fix: handle missing vary header values #15893

Job	Run time
dependency-review	12s
Lint	41s
Fuzzing	3m 32s
Test with SQLite enabled	26s
Test TypeScript types	38s
Test with Node.js 20 compiled --without-intl	4m 47s
Test with Node.js 22 compiled --without-intl	4m 29s
Test with Node.js 23 compiled --without-intl	4m 31s
test (20, macos-latest) / Test with Node.js 20 on macos-latest	4m 39s
test (20, ubuntu-latest) / Test with Node.js 20 on ubuntu-latest	3m 44s
test (22, macos-latest) / Test with Node.js 22 on macos-latest	3m 35s
test (20, windows-latest) / Test with Node.js 20 on windows-latest	5m 24s
test (22, ubuntu-latest) / Test with Node.js 22 on ubuntu-latest	4m 7s
test (22, windows-latest) / Test with Node.js 22 on windows-latest	5m 12s
test (23, macos-latest) / Test with Node.js 23 on macos-latest	3m 11s
test (23, ubuntu-latest) / Test with Node.js 23 on ubuntu-latest	3m 26s
test (23, windows-latest) / Test with Node.js 23 on windows-latest	3m 2s
automerge	0s
	55m 36s

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