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]

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See also:

• Other pull requests
• Contribution Guidelines - Contributing Overview