Optimize NULL handling for string concatentation #21534

Is your feature request related to a problem or challenge?

When concatenating (||) two Utf8Views, for every row we check two Options and build the NULL bitmap incrementally. It would probably be faster to build the NULL bitmap in bulk, which would mean checking a single bitmap per row instead of examining both Options. Annoyingly, StringViewBuilder doesn't have a way to use a precomputed null bitmap, so we still need to build the null bitmap itself incrementally.

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