RFR: 8324286: Fix backsliding on use of nullptr instead of NULL

[Kim Barrett]

Please review this change that removes some new (since JDK-8299837) uses of
NULL in HotSpot code.  Most of the changes are in comments, replacing "NULL"
with either "nullptr" (for code snippets) or "null" (for textual description),
as was done for JDK-8299837.  There are a small number of new uses of NULL in
code, which are replaced with nullptr.

Testing: mach5 tier1

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Commit messages:
 - fix backsliding on NULL usage

Changes: https://git.openjdk.org/jdk/pull/17516/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17516&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8324286
  Stats: 19 lines in 9 files changed: 0 ins; 0 del; 19 mod
  Patch: https://git.openjdk.org/jdk/pull/17516.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17516/head:pull/17516

PR: https://git.openjdk.org/jdk/pull/17516