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

[Johan Sjölen]

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.

I consider this a trivial review. I'd appreciate it if the reviewer concurs so that I can merge it with only 1 reviewer.

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