[crac] RFR: 8358956: [CRaC] Fix NULL usage in hotspot

[Dmitry Cherepanov]

jtreg test TestNoNULL.java fails due to a number of usages of NULL in CRaC code.

The test becomes a part of hs/tier1_common after [JDK-8352645](https://bugs.openjdk.org/browse/JDK-8352645)  (integrated in b17) and GHA starts reporting the failures after merge with jdk-25+17.

The patch replaces NULL with nullptr (and with null in comments and messages in asserts). The test passes with the change.

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Commit messages:
 - fix NULL usage
 - enable check

Changes: https://git.openjdk.org/crac/pull/237/files
  Webrev: https://webrevs.openjdk.org/?repo=crac&pr=237&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8358956
  Stats: 36 lines in 13 files changed: 1 ins; 4 del; 31 mod
  Patch: https://git.openjdk.org/crac/pull/237.diff
  Fetch: git fetch https://git.openjdk.org/crac.git pull/237/head:pull/237

PR: https://git.openjdk.org/crac/pull/237