RFR: 8306459: s390x: Replace NULL to nullptr

[Amit Kumar]

This PR changes two occurrences of NULL to null & nullptr. It is a trivial change OR probably we can say some left out from #12325.

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Commit messages:
 - replace NULL with nullptr

Changes: https://git.openjdk.org/jdk/pull/13538/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13538&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8306459
  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/13538.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13538/head:pull/13538

PR: https://git.openjdk.org/jdk/pull/13538