feat: Add ignore_nulls option to struct.json_encode

[nikhilramolla]

Feature issue: #17865

Adding an option to ignore null values inside a struct when performing json_encode.

[codecov]

Codecov Report

Attention: Patch coverage is 89.47368% with 4 lines in your changes missing coverage. Please review.

Project coverage is 80.56%. Comparing base (3016c07) to head (045b7f6).
Report is 338 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-json/src/json/write/serialize.rs	88.23%	2 Missing ⚠️
crates/polars-json/src/json/write/mod.rs	0.00%	1 Missing ⚠️
...rates/polars-plan/src/dsl/function_expr/struct_.rs	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17867      +/-   ##
==========================================
+ Coverage   80.47%   80.56%   +0.08%     
==========================================
  Files        1505     1495      -10     
  Lines      197273   197025     -248     
  Branches     2807     2813       +6     
==========================================
- Hits       158764   158727      -37     
+ Misses      37987    37777     -210     
+ Partials      522      521       -1     

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[tim-stephenson]

Any updates on this? I was working on adding json_encode to the root namespace (so everything can be json_encode-able). In doing so I wanted to think about adding the ignore_nulls option, and how to add it to all the arrow types (not just struct). Is there a better method to accomplish this than adding it to all the functions in crates/polars-json/src/json/write/serialize.rs?

[nikhilramolla]

@tim-stephenson since the json serialization at crates/polars-json/src/json/write/serialize.rs is handwritten, the ignore_nulls option had to be added to skip wherever needed.

Moving json_encode to the root namespace would be way better. We could use iterators which can optionally skip null values, and serialize with serde.