RFR: 8308092: Replace NULL with nullptr in gc/x

[Thomas Schatzl]

On Mon, 15 May 2023 11:40:26 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

> Replace NULL with nullptr in gc/x. We've already done this work for Generational ZGC, but left it for the Singlegen ZGC code.

Marked as reviewed by tschatzl (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/13984#pullrequestreview-1426415274