RFR: 8327173: HotSpot Style Guide needs update regarding nullptr vs NULL

[Kim Barrett]

Please review this change to update the HotSpot Style Guide's discussion of
nullptr and its use.

I suggest this is an editorial rather than substantive change to the style
guide.  As such, the normal HotSpot PR process can be used for this change.

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Commit messages:
 - update nullptr usage

Changes: https://git.openjdk.org/jdk/pull/18101/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18101&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8327173
  Stats: 19 lines in 2 files changed: 10 ins; 0 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/18101.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18101/head:pull/18101

PR: https://git.openjdk.org/jdk/pull/18101