RFR: 8336085: Fix simple -Wzero-as-null-pointer-constant warnings in CDS code

[Kim Barrett]

Please review this trivial change that replaces some uses of literal 0 as a
null pointer constant in cds code to instead use nullptr.

Testing: mach5 tier1

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Commit messages:
 - CDSMapLogger
 - cds-filemap

Changes: https://git.openjdk.org/jdk/pull/20108/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20108&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8336085
  Stats: 5 lines in 2 files changed: 0 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/20108.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20108/head:pull/20108

PR: https://git.openjdk.org/jdk/pull/20108