[jdk17u-dev] RFR: 8288109: HttpExchangeImpl.setAttribute does not allow null value after JDK-8266897

[Goetz Lindenmaier]

I backport this for parity with 17.0.18-oracle.

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Commit messages:
 - Backport f9827ad17205ad1cec21bf76f8553f415439b38b

Changes: https://git.openjdk.org/jdk17u-dev/pull/4012/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=4012&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8288109
  Stats: 120 lines in 2 files changed: 119 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/4012.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/4012/head:pull/4012

PR: https://git.openjdk.org/jdk17u-dev/pull/4012