RFR: JDK-8301069: Replace NULL with nullptr in share/libadt/

[Vladimir Kozlov]

On Wed, 25 Jan 2023 11:45:35 GMT, Johan Sjölen <jsjolen at openjdk.org> wrote:

> Do the conversion in the share/libadt/ sub-directory and all of its files.

Good and trivial.

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Marked as reviewed by kvn (Reviewer).

PR: https://git.openjdk.org/jdk/pull/12184