RFR: 8335592: Fix -Wzero-as-null-pointer-constant warnings in RootNode ctor

[Kim Barrett]

Please review this trivial change to the RootNode constructor. Both arguments
to the LoopNode base class constructor are changed from 0 to nullptr. This
removes some -Wzero-as-null-pointer-constant warnings when building with that
enabled.

Testing: mach5 tier1

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Commit messages:
 - fix RootNode ctor call to LoopNode ctor

Changes: https://git.openjdk.org/jdk/pull/19997/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19997&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335592
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/19997.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19997/head:pull/19997

PR: https://git.openjdk.org/jdk/pull/19997