prevent expensive type coercion in expression and fix when->then->oth…

[ritchie46]

…eriwise fill null for list

[codecov-commenter]

Codecov Report

Merging #3579 (a78d7d9) into master (fe771f5) will increase coverage by 0.02%.
The diff coverage is 64.00%.

@@            Coverage Diff             @@
##           master    #3579      +/-   ##
==========================================
+ Coverage   61.55%   61.58%   +0.02%     
==========================================
  Files         427      427              
  Lines       71472    71512      +40     
==========================================
+ Hits        43995    44038      +43     
+ Misses      27477    27474       -3     

Impacted Files	Coverage Δ
polars/polars-lazy/src/logical_plan/format.rs	20.89% <0.00%> (ø)
py-polars/polars/internals/expr.py	95.67% <ø> (ø)
polars/polars-lazy/src/logical_plan/lit.rs	45.07% <35.29%> (-3.08%)	⬇️
polars/polars-core/src/datatypes.rs	46.79% <53.84%> (-0.02%)	⬇️
polars/polars-lazy/src/dsl/list.rs	90.36% <100.00%> (ø)
...s-lazy/src/logical_plan/optimizer/type_coercion.rs	80.86% <100.00%> (-0.42%)	⬇️
.../polars-lazy/src/physical_plan/expressions/cast.rs	50.00% <100.00%> (+5.26%)	⬆️
py-polars/polars/internals/whenthen.py	97.14% <100.00%> (ø)
...lars/polars-core/src/chunked_array/builder/list.rs	60.17% <0.00%> (-1.72%)	⬇️
... and 14 more

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