RFR: 8324286: Fix backsliding on use of nullptr instead of NULL [v2]

[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

Kim Barrett has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains two additional commits since the last revision:

 - Merge branch 'master' into cleanup-null
 - fix backsliding on NULL usage

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/17516/files
  - new: https://git.openjdk.org/jdk/pull/17516/files/0ce29eed..bd53e8b6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17516&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17516&range=00-01

  Stats: 488 lines in 37 files changed: 298 ins; 38 del; 152 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