Support nullary RowFn execution

[connortsui20]

Rationale for this change

• Tracking issue: Tracking Issue: Execute RowFn over Vortex arrays #9130
• Epic: Epic: Row-oriented scalar functions #9128

Keeps nullary execution separate from the numeric-kernel executor path.

What changes are included in this PR?

Executes a nullary row kernel once for the requested row count, then validates and returns its output. Nullary kernels bypass batch validity handling because they have no input validity to propagate.

What APIs are changed? Are there any user-facing changes?

Adds nullary execution within the existing unstable RowFn boundary. There are no stable API changes.

[codspeed-hq]

Merging this PR will degrade performance by 10.52%

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

❌ 1 regressed benchmark
✅ 1980 untouched benchmarks
⏩ 442 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	compress_fsst[(500, 64, 4)]	462.4 µs	516.8 µs	-10.52%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing ct/row-fn-nullary (a9cfe2f) with develop (46a8d39)}

Footnotes

1. 442 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩