[fix](exec) preserve NULL in pushed-down CHAR MIN/MAX

[Mryange]

What problem does this PR solve?

Problem Summary: MIN on a nullable CHAR column could return an empty string when string min/max aggregation was pushed down to multiple tablets. The zone-map reader produced NULL correctly, but the CHAR padding cleanup rebuilt each value through a non-null Field, converting StringRef{nullptr, 0} into an empty string. Preserve the original StringRef through insert_data() so NULL remains NULL while CHAR padding is still removed.

Release note

Fix incorrect MIN results for nullable CHAR columns when min/max aggregation pushdown is enabled.

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.
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[Mryange]

/review

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