Fix null count when casting ListArray

[alamb]

Which issue does this PR close?

Closes #815

Rationale for this change

The null count of the output of cast is incorrect for ListArrays

What changes are included in this PR?

Fix null calculation

Are there any user-facing changes?

Bug fixed

[alamb]

this is the fix (should use the null count from the original array rather than the values)

[alamb]

This is also consistent with arrow2: https://github.com/jorgecarleitao/arrow2/blob/a15044a1bc0c079c40520ad5e91b240998b3ee0a/src/compute/cast/mod.rs#L282

cc @jorgecarleitao

[codecov-commenter]

Codecov Report

Merging #816 (317d50a) into master (b9384f1) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 317d50a differs from pull request most recent head fe2c1f3. Consider uploading reports for the commit fe2c1f3 to get more accurate results

@@            Coverage Diff             @@
##           master     #816      +/-   ##
==========================================
- Coverage   82.54%   82.54%   -0.01%     
==========================================
  Files         168      168              
  Lines       47919    47910       -9     
==========================================
- Hits        39555    39546       -9     
  Misses       8364     8364              

Impacted Files	Coverage Δ
arrow/src/compute/kernels/cast.rs	94.60% <100.00%> (-0.03%)	⬇️

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[nevi-me]

LGTM, the change makes sense. Thanks @alamb