refactor(bvar): replace NULL with nullptr

[darion-yaphet]

What problem does this PR solve?

Issue Number: resolve N/A

Problem Summary:

src/bvar still used the legacy NULL macro for pointer null values. Since bRPC already builds with at least C++11, using nullptr is clearer and type-safe, especially in overloaded calls and pointer comparisons.

What is changed and the side effects?

Changed:

• Replaced NULL with nullptr across src/bvar headers and implementation files.
• Updated related comments that referred to returning NULL.
• Kept the cleanup scoped to bvar sources only.

Side effects:

• Performance effects: None expected. This is a source-level modernization with no intended runtime behavior change.

• Breaking backward compatibility: No. nullptr is available under the project’s C++11 baseline.

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Check List:

• Please make sure your changes are compilable.
  • Verified with cmake --build build --target brpc-static -j6.
  • Verified with git diff --check.
  • Verified no NULL tokens remain in src/bvar via rg '\bNULL\b' src/bvar -g '*.{h,cpp}'.
• When providing us with a new feature, it is best to add related tests.
  • Not applicable; this PR does not add a feature or change behavior.
• Please follow Contributor Covenant Code of Conduct.

[chenBright]

Nice job!

I previously submitted a PR (#2786) to refactor all NULL values ​​in the project into nullptr. However, the change was too significant, so it wasn't merged.

I think it could be split into multiple PRs for the refactoring.

[chenBright]

LGTM