test: specify three-valued NULL semantics across the engine - #51
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Pins down NULL behaviour as an explicit specification, independent of the generated differential tests:
WHEREkeeping only exactly-true rows,NOT unknownstaying excluded,IS [NOT] NULL,NULL = NULLnot being true, arithmetic yielding NULL, joins dropping NULL keys,COUNT(col)vsCOUNT(*), an all-NULL average, andGROUP BYcollapsing NULL keys into one group — the one place SQL treats NULL as a value.All ten pass against the current engine, so this documents existing behaviour rather than changing it.
Closes #43