RFR: 8335294: Fix simple -Wzero-as-null-pointer-constant warnings in gc code

[Kim Barrett]

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

There is also one place where the use was eliminated entirely, because it was
dead code following a [[noreturn]] call.

Testing: mach5 tier1.

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Commit messages:
 - simple fixes in gc code

Changes: https://git.openjdk.org/jdk/pull/19934/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19934&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335294
  Stats: 31 lines in 10 files changed: 0 ins; 1 del; 30 mod
  Patch: https://git.openjdk.org/jdk/pull/19934.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19934/head:pull/19934

PR: https://git.openjdk.org/jdk/pull/19934