[I] Avoid a copy in `Array::logical_nulls` [arrow-rs]

["alamb (via GitHub)" <gi...@apache.org>]

alamb opened a new issue, #5208:
URL: https://github.com/apache/arrow-rs/issues/5208

   **Is your feature request related to a problem or challenge? Please describe what you are trying to do.**
   Some code requires "logical nullability" which is distinct from the "physical" nullability (what is defined by an Arrow arrays NullBuffer, if it has one)
   
   To this end, arrow includes `Array::logical_nulls` which will compute logical nulls.
   
   The issue is that it also potentially copies the `NullBuffer` for arrays that have it pre-computed, such as `PrimitiveArray` and `StringArray`. While the copy is perhaps not that bad (it is several `Arc`s) I would like to avoid it entirely if possible
   
   **Describe the solution you'd like**
   I would like to change `logical_nulls` to return a `Cow` to avoid the copy, as I propose in https://github.com/coralogix/arrow-datafusion/pull/221
   
   
   **Describe alternatives you've considered**
   We can not make any changes
   
   **Additional context**
   Thus came up in DataFusion PR https://github.com/apache/arrow-datafusion/pull/8511 
   
   Logical nulls was added in https://github.com/apache/arrow-rs/pull/4691 
   
   


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