RFR: 8306459: s390x: Replace NULL to nullptr

[Martin Doerr]

On Wed, 19 Apr 2023 16:01:22 GMT, Amit Kumar <amitkumar at openjdk.org> wrote:

> 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.

LGTM.

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

Marked as reviewed by mdoerr (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/13538#pullrequestreview-1393661147