[fix](union) fix distribute exprs of union operator

[Gabriel39]

What problem does this PR solve?

Union operator picks a wrong distribute expr now.

related PR: #56157

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]

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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.
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.
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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

[github-actions]

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

[github-actions]

PR approved by anyone and no changes requested.

[doris-robot]

BE UT Coverage Report

Increment line coverage 0.00% (0/5) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	52.39% (17613/33616)
Line Coverage	37.62% (160033/425401)
Region Coverage	32.16% (122043/379431)
Branch Coverage	33.51% (53527/159716)

[hello-stephen]

BE Regression && UT Coverage Report

Increment line coverage 100.00% (5/5) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	71.16% (23356/32820)
Line Coverage	57.55% (244145/424246)
Region Coverage	52.80% (202980/384462)
Branch Coverage	54.49% (87376/160365)

[doris-robot]

ClickBench: Total hot run time: 30.22 s

machine: 'aliyun_ecs.c7a.8xlarge_32C64G'
scripts: https://github.com/apache/doris/tree/master/tools/clickbench-tools
ClickBench test result on commit b5c5a647e1c2db2c732347ed3b54931b23722c55, data reload: false

query1	0.06	0.06	0.05
query2	0.09	0.06	0.06
query3	0.26	0.09	0.09
query4	1.62	0.11	0.12
query5	0.28	0.27	0.26
query6	1.22	0.66	0.66
query7	0.03	0.03	0.02
query8	0.05	0.04	0.04
query9	0.63	0.55	0.53
query10	0.58	0.62	0.58
query11	0.17	0.11	0.12
query12	0.15	0.12	0.12
query13	0.63	0.64	0.61
query14	1.03	1.02	1.04
query15	0.90	0.88	0.87
query16	0.41	0.40	0.41
query17	1.03	1.05	1.05
query18	0.22	0.20	0.20
query19	1.97	1.86	1.83
query20	0.02	0.01	0.02
query21	15.41	0.90	0.58
query22	0.76	1.24	0.65
query23	14.94	1.37	0.64
query24	7.12	1.30	0.68
query25	0.50	0.11	0.08
query26	0.66	0.16	0.14
query27	0.07	0.06	0.06
query28	9.22	1.40	0.94
query29	12.55	3.86	3.22
query30	0.28	0.15	0.12
query31	2.84	0.60	0.39
query32	3.23	0.56	0.49
query33	3.15	3.00	3.20
query34	16.22	5.47	4.83
query35	4.94	4.93	4.87
query36	0.71	0.53	0.52
query37	0.10	0.07	0.07
query38	0.07	0.05	0.04
query39	0.05	0.03	0.03
query40	0.18	0.15	0.16
query41	0.09	0.04	0.03
query42	0.04	0.03	0.04
query43	0.05	0.04	0.04
Total cold run time: 104.53 s
Total hot run time: 30.22 s