[fix](flinksql): fix elemet_at may coredump on arm

[yangzhg]

What problem does this PR solve?

fix elemet_at may coredump on arm

Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 🚀 Performance improvement (optimization)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)
• 🔨 Refactoring (no logic changes)
• 🔧 Build/CI or Infrastructure changes
• 📝 Documentation only

Description

This pull request focuses on improving the test coverage and error handling for the element_at function in Flink SQL, as well as making a minor fix in the SubscriptUtil utility. The main changes include adding a new test case to verify exception handling when a zero index is used across multiple rows, and ensuring proper creation of null constants in the utility code.

Performance Impact

• No Impact: This change does not affect the critical path (e.g., build system, doc, error handling).

• Positive Impact: I have run benchmarks.

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  Before: 10.5s
  After:   8.2s  (+20%)
  

• Negative Impact: Explained below (e.g., trade-off for correctness).

Release Note

Please describe the changes in this PR

Release Note:

Release Note:
- Fixed a crash in `elemet_at `.

Checklist (For Author)

• I have added/updated unit tests (ctest).
• I have verified the code with local build (Release/Debug).
• I have run clang-format / linters.
• (Optional) I have run Sanitizers (ASAN/TSAN) locally for complex C++ changes.
• No need to test or manual test.

Breaking Changes

• No

• Yes (Description: ...)

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  Breaking Changes:
  - Description of the breaking change.
  - Possible solutions or workarounds.
  - Any other relevant information.
  

[Copilot]

Pull request overview

This PR addresses a crash in Flink SQL element_at on ARM by ensuring a valid (null constant) result vector is produced when a constant invalid index triggers an error, and it adds a regression test for the multi-row constant-zero-index case.

Changes:

• Return a null-constant vector early when constant index validation fails for arrays in SubscriptUtil.
• Add a new Flink SQL element_at unit test covering constant index 0 across multiple rows.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
bolt/functions/lib/SubscriptUtil.h	Early-return a null constant vector for constant invalid index error cases to avoid undefined behavior/crash.
bolt/functions/flinksql/tests/ElementAtTest.cpp	Adds a multi-row regression test for constant zero index throwing.

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[Copilot]

Pull request overview

Fixes a crash in FlinkSQL element_at (notably on ARM) when a constant invalid index (e.g., 0) is applied across rows, by ensuring the array subscript path returns a correctly typed null constant vector when all rows are marked failed, and adds a regression test for the multi-row constant-zero-index case.

Changes:

• Return a typed null constant vector from SubscriptImpl::applyArrayTyped when a constant index triggers a zero-subscript error for all selected rows.
• Add a new FlinkSQL element_at unit test covering constant index 0 across multiple input rows.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
bolt/functions/lib/SubscriptUtil.h	Prevents using uninitialized dictionary indices when constant index validation fails for all rows by returning a null constant vector early.
bolt/functions/flinksql/tests/ElementAtTest.cpp	Adds a regression test to ensure constant 0 index throws consistently for multi-row inputs.

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