feat: add json null serialization

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.04 :warning:

Comparison is base (2d2e705) 83.47% compared to head (84f06d6) 83.44%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1512 +/- ## ========================================== - Coverage 83.47% 83.44% -0.04% ========================================== Files 388 388 Lines 41996 42019 +23 ========================================== + Hits 35057 35062 +5 - Misses 6939 6957 +18 ``` | [Impacted Files](https://app.codecov.io/gh/jorgecarleitao/arrow2/pull/1512?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jorge+Leitao) | Coverage Δ | | |---|---|---| | [src/io/json/write/serialize.rs](https://app.codecov.io/gh/jorgecarleitao/arrow2/pull/1512?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jorge+Leitao#diff-c3JjL2lvL2pzb24vd3JpdGUvc2VyaWFsaXplLnJz) | `91.71% <100.00%> (+0.23%)` | :arrow_up: | ... and [5 files with indirect coverage changes](https://app.codecov.io/gh/jorgecarleitao/arrow2/pull/1512/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jorge+Leitao)

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