Do not write validity without nulls in IPC

[jorgecarleitao]

Writing one is not prohibited, but smaller files are always better ^^

[codecov]

Codecov Report

Merging #688 (bae2f75) into main (565cfae) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #688      +/-   ##
==========================================
- Coverage   69.77%   69.77%   -0.01%     
==========================================
  Files         303      303              
  Lines       16830    16829       -1     
==========================================
- Hits        11743    11742       -1     
  Misses       5087     5087              

Impacted Files	Coverage Δ
src/io/ipc/write/serialize.rs	64.66% <100.00%> (-0.13%)	⬇️

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