fix take kernel null handling on structs

[bjchambers]

This closes #530.

[codecov-commenter]

Codecov Report

Merging #531 (4342d28) into master (f1a831f) will decrease coverage by 0.15%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #531      +/-   ##
==========================================
- Coverage   82.74%   82.59%   -0.16%     
==========================================
  Files         165      167       +2     
  Lines       45686    45975     +289     
==========================================
+ Hits        37805    37974     +169     
- Misses       7881     8001     +120     

Impacted Files	Coverage Δ
arrow/src/compute/kernels/take.rs	94.72% <100.00%> (+0.64%)	⬆️
arrow/src/array/transform/boolean.rs	76.92% <0.00%> (-7.70%)	⬇️
parquet/src/encodings/encoding.rs	94.85% <0.00%> (-0.20%)	⬇️
parquet/src/arrow/arrow_writer.rs	97.98% <0.00%> (-0.07%)	⬇️
parquet/src/util/bit_packing.rs	99.96% <0.00%> (-0.01%)	⬇️
arrow-flight/src/utils.rs	0.00% <0.00%> (ø)
arrow/src/datatypes/mod.rs	100.00% <0.00%> (ø)
arrow/src/array/equal/variable_size.rs	100.00% <0.00%> (ø)
arrow-pyarrow-integration-testing/src/lib.rs	0.00% <0.00%> (ø)
...ng/src/flight_client_scenarios/integration_test.rs	0.00% <0.00%> (ø)
... and 13 more

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