RFR: 8324679: Replace NULL with nullptr in HotSpot .ad files

[Coleen Phillimore]

This is a straightforward mechanical (sed) replacement of NULL with nullptr.  One string was adjusted to not say nullptr ptr.  nullptr makes sense in the comments so I didn't change them.

Tested with tier1 Oracle platforms and build test with linux-x64-zero,linux-x64-zero-debug,linux-s390x-open,linux-arm32-debug,linux-ppc64le-debug,linux-riscv64-debug

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Commit messages:
 - 8324679: Replace NULL with nullptr in HotSpot .ad files

Changes: https://git.openjdk.org/jdk/pull/17658/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17658&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8324679
  Stats: 327 lines in 9 files changed: 0 ins; 0 del; 327 mod
  Patch: https://git.openjdk.org/jdk/pull/17658.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17658/head:pull/17658

PR: https://git.openjdk.org/jdk/pull/17658