Validity taken into account when writing StructArray to json

[VasanthakumarV]

Close #500

[codecov]

Codecov Report

Merging #511 (ddf2e9f) into main (8a7f1d7) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #511      +/-   ##
==========================================
+ Coverage   80.17%   80.20%   +0.02%     
==========================================
  Files         381      381              
  Lines       23659    23722      +63     
==========================================
+ Hits        18969    19026      +57     
- Misses       4690     4696       +6     

Impacted Files	Coverage Δ
src/io/json/write/serialize.rs	63.41% <100.00%> (+1.15%)	⬆️
tests/it/io/json/write.rs	100.00% <100.00%> (ø)
src/io/parquet/read/statistics/fixlen.rs	39.53% <0.00%> (-4.66%)	⬇️
src/io/ipc/read/reader.rs	82.20% <0.00%> (-1.42%)	⬇️
src/io/parquet/read/mod.rs	59.59% <0.00%> (-0.41%)	⬇️
src/io/parquet/read/schema/convert.rs	91.26% <0.00%> (-0.05%)	⬇️
src/array/ffi.rs	65.00% <0.00%> (ø)
src/array/list/mutable.rs	66.33% <0.00%> (ø)
src/compute/arithmetics/decimal/add.rs	83.09% <0.00%> (ø)
src/compute/arithmetics/decimal/div.rs	81.01% <0.00%> (ø)
... and 10 more

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[VasanthakumarV]

@jorgecarleitao Should nullable from Field influence StructArray's validity?

[jorgecarleitao]

Interesting approach. I would imagine that we do not need to combine the validities for this, and instead just write a null to the whole field, but maybe you already went through that exercise and deemed not feasible?

If yes, could you clarify this on the code with a comment? If not, I think it is worth the shot, since it would make the file smaller

[jorgecarleitao]

This looks awesome, and very clean with the zip_validity 💯

Left two nits that clippy would complain.