fix: avoid map_from_entries error on empty array(unknown)

[zjuwangg]

For Presto, map_from_entries on array(unknown) previously marked all selected rows as errored, even rows whose array was empty. Empty arrays should produce empty maps without raising "map entry cannot be null". Only flag the error on rows whose array size is greater than zero.

What problem does this PR solve?

Issue Number: close #xxx

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

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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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  Paste your google-benchmark or TPC-H results here.
  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 `substr` when input is null.
- optimized `group by` performance by 20%.

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

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  Breaking Changes:
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[zjuwangg]

cherry-pick from facebookincubator/velox@48cb6b1