RFR: 8324678: Replace NULL with nullptr in HotSpot gtests [v3]

[Coleen Phillimore]

> If this is sufficient, here's the change for NULL to nullptr, adjusting some obvious strings that had NULL in them maybe not all.
> Tested with gtest,

Coleen Phillimore has updated the pull request incrementally with two additional commits since the last revision:

 - Removed some casts unneeded by nullptr change, and fixed one comment
 - Removed some casts unneeded by nullptr change, and fixed one comment

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Changes:
  - all: https://git.openjdk.org/jdk/pull/17577/files
  - new: https://git.openjdk.org/jdk/pull/17577/files/d8e3d92d..cf30de3d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17577&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17577&range=01-02

  Stats: 25 lines in 20 files changed: 0 ins; 0 del; 25 mod
  Patch: https://git.openjdk.org/jdk/pull/17577.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17577/head:pull/17577

PR: https://git.openjdk.org/jdk/pull/17577