fix(proxy-agent): add fallback value for servername when using reques… #15807

Job	Run time
dependency-review	10s
Lint	35s
Fuzzing	3m 30s
Test with SQLite enabled	24s
Test TypeScript types	35s
Test with Node.js 20 compiled --without-intl	5m 0s
Test with Node.js 22 compiled --without-intl	5m 4s
Test with Node.js 23 compiled --without-intl	4m 54s
test (20, macos-latest) / Test with Node.js 20 on macos-latest	5m 2s
test (20, windows-latest) / Test with Node.js 20 on windows-latest	5m 55s
test (22, macos-latest) / Test with Node.js 22 on macos-latest	3m 55s
test (20, ubuntu-latest) / Test with Node.js 20 on ubuntu-latest	4m 16s
test (22, ubuntu-latest) / Test with Node.js 22 on ubuntu-latest	4m 38s
test (22, windows-latest) / Test with Node.js 22 on windows-latest	5m 58s
test (23, macos-latest) / Test with Node.js 23 on macos-latest	3m 44s
test (23, ubuntu-latest) / Test with Node.js 23 on ubuntu-latest	4m 2s
test (23, windows-latest) / Test with Node.js 23 on windows-latest	5m 26s
automerge	0s
	1h 3m 8s

Provide feedback