MINOR: [C++] Replace NULLPTR with nullptr inside registry.cc

[andishgar]

Rationale for this change

As mentioned here, NULLPTR must be used in headers. However, NULLPTR is used inside arrow/compute/registry.cc instead of nullptr.

What changes are included in this PR?

Replace NULLPTR with nullptr.

Are these changes tested?

Yes, I ran the unit tests.

Are there any user-facing changes?

No.

[github-actions]

Thanks for opening a pull request!

If this is not a minor PR. Could you open an issue for this pull request on GitHub? https://github.com/apache/arrow/issues/new/choose

Opening GitHub issues ahead of time contributes to the Openness of the Apache Arrow project.

Then could you also rename the pull request title in the following format?

GH-${GITHUB_ISSUE_ID}: [${COMPONENT}] ${SUMMARY}

or

MINOR: [${COMPONENT}] ${SUMMARY}

See also:

• Other pull requests
• Contribution Guidelines - Contributing Overview

[pitrou]

@kou Do you think this is ok as a minor PR?

[kou]

+1

This should not be a MINOR PR for us: https://github.com/apache/arrow/blob/main/CONTRIBUTING.md#Minor-Fixes

Could you create an issue next time?

[andishgar]

Thanks, @kou. I will consider your advice.

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 4 benchmarking runs that have been run so far on merge-commit ee0d861.

There were no benchmark performance regressions. 🎉

The full Conbench report has more details. It also includes information about 56 possible false positives for unstable benchmarks that are known to sometimes produce them.