RFR: 8327171: Fix more NULL usage backsliding

[Kim Barrett]

Please review this trivial change that replaces some recently introduced uses
of NULL with nullptr.

Testing: mach5 tier1

-------------

Commit messages:
 - NULL -> nullptr

Changes: https://git.openjdk.org/jdk/pull/18096/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18096&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8327171
  Stats: 5 lines in 3 files changed: 0 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/18096.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18096/head:pull/18096

PR: https://git.openjdk.org/jdk/pull/18096