[fix](test) Unset Paimon Variant shredding property

[Gabriel39]

What problem does this PR solve?

Issue Number: N/A

Related PR: #66503

Problem Summary:

Paimon 1.4.2 parses every present Variant shredding schema property as JSON. The Paimon Variant fixture attempted to disable shredding by setting the property to an empty string, so the following unshredded insert failed and left the mixed-layout table with only its shredded row.

This change removes the property with UNSET TBLPROPERTIES before writing the unshredded file. It preserves the intended shredded-to-unshredded mixed-file coverage.

Check List

• Test
  • Regression test fixture
  • Manual test

Validation:

• Verified the fixture no longer contains an empty shredding schema property.

• Ran the shredded-to-unshredded write sequence in Spark/Paimon and confirmed both rows were returned.

• git diff --check

• Behavior changed:

  • No production behavior change; this fixes test data generation.

• Does this need documentation?

  • No.

[hello-stephen]

Thank you for your contribution to Apache Doris.
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Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
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4. Which code was refactored and why was this part of the code refactored?
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[Gabriel39]

run buildall

[Gabriel39]

/review

[github-actions]

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[hello-stephen]

skip buildall

[github-actions]

PR approved by at least one committer and no changes requested.

[github-actions]

PR approved by anyone and no changes requested.