RFR: 8342860: Fix more NULL usage backsliding [v3]

[theoweidmannoracle]

> - Changed several "NULL" in comments to "null"
> - Changed several `NULL` in code to `nullptr`

theoweidmannoracle has updated the pull request incrementally with one additional commit since the last revision:

  Fix backslides in test

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/21826/files
  - new: https://git.openjdk.org/jdk/pull/21826/files/9754145b..e79b7bde

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21826&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21826&range=01-02

  Stats: 10 lines in 2 files changed: 0 ins; 0 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/21826.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21826/head:pull/21826

PR: https://git.openjdk.org/jdk/pull/21826