Improve memory accounting for ArrowBytesViewMap

[alamb]

I think this should use https://docs.rs/arrow/latest/arrow/array/struct.NullBufferBuilder.html#method.allocated_size as it is meant to track the allocated size (not the used size)

Originally posted by @alamb in #20064 (comment)

[alamb]

This is a small change that will help with memory accounting accuracy -- it would be a good exercise for someone to get used to making PRs and running tests

[vigneshsiva11]

Hi @alamb, I'd like to work on this issue.

[vigneshsiva11]

I’ve opened PR #20077 to address this issue.