RFR: 8377796: java.net.http.HttpClient.send() and sendAsync() never complete when BodyHandler.apply() returns null [v2]

[Jaikiran Pai]

> Can I please get a review of this change which proposes to address the issue noted in https://bugs.openjdk.org/browse/JDK-8377796?
> 
> The change here asserts that if `BodyHandler.apply()` returns `null`, then the HTTP request completes exceptionally. A new test has been introduced to reproduce the issue and verify the fix.

Jaikiran Pai has updated the pull request incrementally with three additional commits since the last revision:

 - split the test method arguments into individual parts
 - no need for the duplicate null check
 - use HttpTestEchoHandler in the new test

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/29691/files
  - new: https://git.openjdk.org/jdk/pull/29691/files/792461b7..52cd9049

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=29691&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=29691&range=00-01

  Stats: 52 lines in 3 files changed: 3 ins; 24 del; 25 mod
  Patch: https://git.openjdk.org/jdk/pull/29691.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29691/head:pull/29691

PR: https://git.openjdk.org/jdk/pull/29691