Fix nullability warnings

[sungam3r]

fixes #64

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.50%. Comparing base (9843575) to head (a0fc509).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Destructurama.FSharp/DestructuramaFSharp.fs	0.00%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #96      +/-   ##
==========================================
- Coverage   70.83%   62.50%   -8.34%     
==========================================
  Files           1        1              
  Lines          24       24              
  Branches        4        6       +2     
==========================================
- Hits           17       15       -2     
  Misses          5        5              
- Partials        2        4       +2     

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