[fix](be) Inject debug payloads into nullable columns

[Mryange]

What problem does this PR solve?

Issue Number: None

Related PR: None

Problem Summary: Debug execution can accidentally read the nested payload of nullable rows even though the null map marks those rows as NULL. Add a Debug-only Block boundary that COW-detaches nullable columns after validation and injects conspicuous type-specific payloads into NULL rows, while preserving the null map and column structure.

Release note

None

Check List (For Author)

• Test: BE ASAN unit test: BlockCheckType.*

• Behavior changed: No in Release builds; Debug builds gain nullable payload validation coverage.

• Does this need documentation: No

What problem does this PR solve?

Issue Number: close #xxx

Related PR: #xxx

Problem Summary:

Release note

None

Check List (For Author)

• Test

  • Regression test
  • Unit Test
  • Manual test (add detailed scripts or steps below)
  • No need to test or manual test. Explain why:
    • This is a refactor/code format and no logic has been changed.
    • Previous test can cover this change.
    • No code files have been changed.
    • Other reason

• Behavior changed:

  • No.
  • Yes.

• Does this need documentation?

  • No.
  • Yes.

Check List (For Reviewer who merge this PR)

• Confirm the release note
• Confirm test cases
• Confirm document
• Add branch pick label

[hello-stephen]

Thank you for your contribution to Apache Doris.
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1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[Mryange]

run buildall