[fix](variant) return raw string for element_at on scalar-string variant (#64103)

[csun5285]

cherry picked from #64103

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.
Don't know what should be done next? See How to process your PR.

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.
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?

[csun5285]

run buildall

[hello-stephen]

BE UT Coverage Report

Increment line coverage 80.00% (4/5) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	53.85% (20389/37861)
Line Coverage	37.45% (193456/516608)
Region Coverage	33.87% (150996/445783)
Branch Coverage	34.84% (66051/189568)

[hello-stephen]

BE Regression && UT Coverage Report

Increment line coverage 80.00% (4/5) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	71.73% (26563/37030)
Line Coverage	54.84% (282248/514690)
Region Coverage	52.27% (235034/449683)
Branch Coverage	53.53% (101737/190055)

[hello-stephen]

BE Regression && UT Coverage Report

Increment line coverage 80.00% (4/5) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	71.73% (26563/37030)
Line Coverage	54.84% (282248/514690)
Region Coverage	52.27% (235034/449683)
Branch Coverage	53.53% (101737/190055)

[hello-stephen]

BE Regression && UT Coverage Report

Increment line coverage 80.00% (4/5) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	71.74% (26564/37030)
Line Coverage	54.85% (282288/514690)
Region Coverage	52.27% (235061/449683)
Branch Coverage	53.54% (101749/190055)