[fix](mtmv) Fix compensate union wrongly when direct query is empty relation (#51700)

[seawinde]

commitId: 85b7231c pr: https://github.com/apache/doris/pull/51700

What problem does this PR solve?

Issue Number: close #xxx

Related PR: #xxx

Problem Summary:

Release note

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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

[seawinde]

run buildall

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[github-actions[bot]]

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

[github-actions[bot]]

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