Optimize CASE WHEN for "expression or null" case

[andygrove]

Is your feature request related to a problem or challenge?

Given the expression CASE WHEN predicate THEN expr ELSE null END (as seen in some TPC-DS queries) we could do better than using our generic CaseExpr. In this specific case we can evaluate expr and create a new array using the existing data buffer but just replace the null mask with the results of evaluating the predicate.

Describe the solution you'd like

No response

Describe alternatives you've considered

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Additional context

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[andygrove]

I am implementing this in Comet in apache/datafusion-comet#672 and plan on upstreaming to DataFusion soon. The optimization is resulting in a > 10x speedup for my test case.