RFR: 8335588: Fix -Wzero-as-null-pointer-constant warnings in calls to Node ctor

[Kim Barrett]

Please review this change to callers of the Node constructor that were passing
0 as the first argument, which is of type Node*, thereby triggering
-Wzero-as-null-pointer-constant warnings when enabled. Instead of 0, pass
nullptr.  This removes over 2K warnings when building with that option.

Testing: mach5 tier1

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Commit messages:
 - fix calls to Node constructor

Changes: https://git.openjdk.org/jdk/pull/19994/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19994&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335588
  Stats: 34 lines in 10 files changed: 0 ins; 0 del; 34 mod
  Patch: https://git.openjdk.org/jdk/pull/19994.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19994/head:pull/19994

PR: https://git.openjdk.org/jdk/pull/19994