adamreeve opened a new pull request, #41144:
URL: https://github.com/apache/arrow/pull/41144

   ### Rationale for this change
   
   See #41136. This is required for writing sliced arrays to IPC files and 
streams. It should also be generally useful too by allowing users to be able to 
rely on valid null count values for sliced arrays.
   
   ### What changes are included in this PR?
   
   Changes `ArrayData.NullCount` to a get-only property that recomputes the 
actual null count on demand if the array data represents a slice of data with 
unknown null count.
   
   ### Are these changes tested?
   
   Yes, I've extended the existing slicing tests to also verify the null count, 
and added new tests specifically for union arrays, which needed special 
handling.
   
   ### Are there any user-facing changes?
   
   Yes, this is a user-facing behaviour change. I don't expect that it should 
break any existing code as users are unlikely to rely on the NullCount being -1.
   
   This doesn't break source compatibility with existing code, but does break 
the ABI for `ArrayData.NullCount`.


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