Skip validity dispatch for non-nullable arrays

[palaska]

Two small optimizations:

• Skip the is_invalid vtable dispatch when dtype is non-nullable
• Demote the dtype-equality post-check to debug assertion. It's an encoding-correctness invariant, not runtime input validation

[codspeed-hq]

Merging this PR will degrade performance by 17.47%

⚠️ 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

⚡ 10 improved benchmarks
❌ 5 regressed benchmarks
✅ 1183 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	WallTime	bitunpack[3bw]	300.5 µs	353.1 µs	-14.9%
⚡	Simulation	case_when_fragmented[1000]	501.8 µs	450.8 µs	+11.32%
⚡	Simulation	extend_from_array_non_zctl_overlapping[(10000, 8)]	6 ms	5.5 ms	+10.18%
⚡	Simulation	execute_scalar_struct_simple	628.9 µs	515.3 µs	+22.06%
⚡	Simulation	execute_scalar_struct_wide	2.6 ms	2.1 ms	+25.18%
❌	Simulation	decompress_rd[f32, (100000, 0.01)]	495.1 µs	582.6 µs	-15.02%
⚡	Simulation	decompress_rd[f64, (10000, 0.0)]	138.5 µs	122.3 µs	+13.27%
❌	Simulation	decompress_rd[f32, (100000, 0.1)]	495.1 µs	582.6 µs	-15.03%
⚡	Simulation	decompress_rd[f32, (100000, 0.0)]	583.5 µs	495.7 µs	+17.72%
⚡	Simulation	decompress_rd[f32, (10000, 0.0)]	94.5 µs	85.7 µs	+10.28%
⚡	Simulation	decompress_rd[f32, (10000, 0.1)]	90.2 µs	81.9 µs	+10.11%
⚡	Simulation	decompress_rd[f64, (10000, 0.01)]	138.6 µs	122.1 µs	+13.53%
❌	Simulation	decompress_rd[f64, (100000, 0.01)]	842.6 µs	1,020.7 µs	-17.45%
❌	Simulation	decompress_rd[f64, (100000, 0.1)]	842.5 µs	1,020.9 µs	-17.47%
⚡	Simulation	decompress_rd[f64, (10000, 0.1)]	138.7 µs	122.2 µs	+13.45%

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_{Comparing bp/shortcut-validity (bbba3ca) with develop (c4feed7)}