Optimize NULL handling in StringViewArrayBuilder #21537

Is your feature request related to a problem or challenge?

StringViewArrayBuilder is implemented on top of Arrow's StringViewBuilder; the latter tracks NULLs incrementally. However, the StringViewArrayBuilder requires callers to pass a NULL buffer to finish() anyway, so the NULL bitmap that has been computed by StringViewBuilder is discarded. It would be more efficient to stop using StringViewBuilder so that we don't do this redundant work; in theory there might be room for inconsistency between the two NULL bitmaps as well.

Describe the solution you'd like

No response

Describe alternatives you've considered

No response

Additional context

No response