fix(python): translate Polars is_null expressions

[danking]

Polars is_null() fails predicate conversion despite native Vortex support. Map the Boolean.IsNull function directly to ve.is_null, including null input rows.

This is a direct expression translation; it performs no eager evaluation and does not require literal arguments. Literal-specific PRs decode the scalar representation itself.

Validation: 1 targeted regression case(s) passed with this PR’s converter. With only polars_.py restored to the base revision and the same tests retained, all 1 failed with Polars ComputeError wrapping the unsupported-expression ValueError or NotImplementedError. The tests compare complete dataframes from Polars alone and a Vortex-backed Polars scan, and check explicit row expectations. No lint or broader suite was run.

[codspeed]

Merging this PR will degrade performance by 15.16%

⚠️ 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.

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 1 improved benchmark
❌ 1 regressed benchmark
✅ 2173 untouched benchmarks
⏩ 534 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	density_sweep_dense_runs[0.001]	30.3 µs	48 µs	-36.94%
⚡	WallTime	bitpack_blocked_compress_avx2	7.6 µs	6.6 µs	+14.15%

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 dk/polars-is-null (61b090a) with develop (fdfe7f4)²}

Footnotes

1. 534 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. ↩

2. No successful run was found on develop (4ba858a) during the generation of this report, so fdfe7f4 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩