mean(all null) = NULL

[myrrc]

#8365 introduced a behavior which is different from ANSI SQL and how query engines implement mean(all null), and reverting in #8389 was closed.

Revert this change, and add SLT duckdb/datafusion tests for current behaviour.

Resolves: #9083

[codspeed-hq]

Merging this PR will degrade performance by 10.31%

❌ 1 regressed benchmark
✅ 1840 untouched benchmarks
⏩ 55 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	fsl_large	88.6 µs	98.8 µs	-10.31%

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 myrrc/mean-all-null-returns-null (1f560bf) with develop (ae00c3b)}

Footnotes

1. 55 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. ↩