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

[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

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Commit messages:
 - Restore `test/jdk/java/net/httpclient/` changes
 - Reverted all changes and only kept `SimpleSSLContext` enhancements
 - Overhaul `SimpleSSLContext` and its usages

Changes: https://git.openjdk.org/jdk/pull/28771/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28771&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373515
  Stats: 1073 lines in 180 files changed: 108 ins; 667 del; 298 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