RFR: 8373515: Migrate "test/jdk/java/net/httpclient/" to null-safe "SimpleSSLContext" methods [v2]

[Volkan Yazici]

> Migrates `test/jdk/java/net/httpclient/` to null-safe `SimpleSSLContext` methods introduced in [JDK-8372661], which is a prerequisite for this change set.
> 
> [JDK-8372661]: https://bugs.openjdk.org/browse/JDK-8372661

Volkan Yazici has updated the pull request incrementally with one additional commit since the last revision:

  Make sure `get()` returns the same instance

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/28771/files
  - new: https://git.openjdk.org/jdk/pull/28771/files/381ae11a..74ec5a69

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28771&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28771&range=00-01

  Stats: 23 lines in 1 file changed: 10 ins; 7 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/28771.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28771/head:pull/28771

PR: https://git.openjdk.org/jdk/pull/28771